Add tests for direction of rounding functionality

The round() and toString() methods of Temporal.Instant, PlainDateTime, and
ZonedDateTime can round up or down. However, the instance must not be
treated as "negative" even when the time is before 1 BCE (years are
negative) or before the Unix epoch (epoch nanoseconds are negative). That
is, rounding down is always towards the Big Bang, and rounding up is
always away from it. Add tests that verify this.

test/built-ins/Temporal/Instant/prototype/round/rounding-direction.js

@@ -0,0 +1,30 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.instant.prototype.round
+description: Rounding down is towards the Big Bang, not the epoch or 1 BCE
+features: [Temporal]
+---*/
+
+const instance = new Temporal.Instant(-65_261_246_399_500_000_000n);  // -000099-12-15T12:00:00.5Z
+assert.sameValue(
+  instance.round({ smallestUnit: "second", roundingMode: "floor" }).epochNanoseconds,
+  -65_261_246_400_000_000_000n,  // -000099-12-15T12:00:00Z
+  "Rounding down is towards the Big Bang, not the epoch or 1 BCE (roundingMode floor)"
+);
+assert.sameValue(
+  instance.round({ smallestUnit: "second", roundingMode: "trunc" }).epochNanoseconds,
+  -65_261_246_400_000_000_000n,  // -000099-12-15T12:00:00Z
+  "Rounding down is towards the Big Bang, not the epoch or 1 BCE (roundingMode trunc)"
+);
+assert.sameValue(
+  instance.round({ smallestUnit: "second", roundingMode: "ceil" }).epochNanoseconds,
+  -65_261_246_399_000_000_000n,  // -000099-12-15T12:00:01Z
+  "Rounding up is away from the Big Bang, not the epoch or 1 BCE (roundingMode ceil)"
+);
+assert.sameValue(
+  instance.round({ smallestUnit: "second", roundingMode: "halfExpand" }).epochNanoseconds,
+  -65_261_246_399_000_000_000n,  // -000099-12-15T12:00:01Z
+  "Rounding up is away from the Big Bang, not the epoch or 1 BCE (roundingMode halfExpand)"
+);

test/built-ins/Temporal/Instant/prototype/toString/rounding-direction.js

@@ -0,0 +1,30 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.instant.prototype.tostring
+description: Rounding down is towards the Big Bang, not the epoch or 1 BCE
+features: [Temporal]
+---*/
+
+const instance = new Temporal.Instant(-65_261_246_399_500_000_000n);  // -000099-12-15T12:00:00.5Z
+assert.sameValue(
+  instance.toString({ smallestUnit: "second", roundingMode: "floor" }),
+  "-000099-12-15T12:00:00Z",
+  "Rounding down is towards the Big Bang, not the epoch or 1 BCE"
+);
+assert.sameValue(
+  instance.toString({ smallestUnit: "second", roundingMode: "trunc" }),
+  "-000099-12-15T12:00:00Z",
+  "Rounding down is towards the Big Bang, not the epoch or 1 BCE (roundingMode trunc)"
+);
+assert.sameValue(
+  instance.toString({ smallestUnit: "second", roundingMode: "ceil" }),
+  "-000099-12-15T12:00:01Z",
+  "Rounding up is away from the Big Bang, not the epoch or 1 BCE (roundingMode ceil)"
+);
+assert.sameValue(
+  instance.toString({ smallestUnit: "second", roundingMode: "halfExpand" }),
+  "-000099-12-15T12:00:01Z",
+  "Rounding up is away from the Big Bang, not the epoch or 1 BCE (roundingMode halfExpand)"
+);

test/built-ins/Temporal/PlainDateTime/prototype/round/rounding-direction.js

@@ -0,0 +1,31 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.round
+description: Rounding down is towards the Big Bang, not the epoch or 1 BCE
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const instance = new Temporal.PlainDateTime(-99, 12, 15, 12, 0, 0, 500);
+TemporalHelpers.assertPlainDateTime(
+  instance.round({ smallestUnit: "second", roundingMode: "floor" }),
+  -99, 12, "M12", 15, 12, 0, 0, 0, 0, 0,
+  "Rounding down is towards the Big Bang, not the epoch or 1 BCE (roundingMode floor)"
+);
+TemporalHelpers.assertPlainDateTime(
+  instance.round({ smallestUnit: "second", roundingMode: "trunc" }),
+  -99, 12, "M12", 15, 12, 0, 0, 0, 0, 0,
+  "Rounding down is towards the Big Bang, not the epoch or 1 BCE (roundingMode trunc)"
+);
+TemporalHelpers.assertPlainDateTime(
+  instance.round({ smallestUnit: "second", roundingMode: "ceil" }),
+  -99, 12, "M12", 15, 12, 0, 1, 0, 0, 0,
+  "Rounding up is away from the Big Bang, not the epoch or 1 BCE (roundingMode ceil)"
+);
+TemporalHelpers.assertPlainDateTime(
+  instance.round({ smallestUnit: "second", roundingMode: "halfExpand" }),
+  -99, 12, "M12", 15, 12, 0, 1, 0, 0, 0,
+  "Rounding up is away from the Big Bang, not the epoch or 1 BCE (roundingMode halfExpand)"
+);

test/built-ins/Temporal/PlainDateTime/prototype/toString/rounding-direction.js

@@ -0,0 +1,30 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.tostring
+description: Rounding down is towards the Big Bang, not the epoch or 1 BCE
+features: [Temporal]
+---*/
+
+const instance = new Temporal.PlainDateTime(-99, 12, 15, 12, 0, 0, 500);
+assert.sameValue(
+  instance.toString({ smallestUnit: "second", roundingMode: "floor" }),
+  "-000099-12-15T12:00:00",
+  "Rounding down is towards the Big Bang, not the epoch or 1 BCE"
+);
+assert.sameValue(
+  instance.toString({ smallestUnit: "second", roundingMode: "trunc" }),
+  "-000099-12-15T12:00:00",
+  "Rounding down is towards the Big Bang, not the epoch or 1 BCE (roundingMode trunc)"
+);
+assert.sameValue(
+  instance.toString({ smallestUnit: "second", roundingMode: "ceil" }),
+  "-000099-12-15T12:00:01",
+  "Rounding up is away from the Big Bang, not the epoch or 1 BCE (roundingMode ceil)"
+);
+assert.sameValue(
+  instance.toString({ smallestUnit: "second", roundingMode: "halfExpand" }),
+  "-000099-12-15T12:00:01",
+  "Rounding up is away from the Big Bang, not the epoch or 1 BCE (roundingMode halfExpand)"
+);

test/built-ins/Temporal/ZonedDateTime/prototype/round/rounding-direction.js

@@ -0,0 +1,30 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.round
+description: Rounding down is towards the Big Bang, not the epoch or 1 BCE
+features: [Temporal]
+---*/
+
+const instance = new Temporal.ZonedDateTime(-65_261_246_399_500_000_000n, "UTC");  // -000099-12-15T12:00:00.5Z
+assert.sameValue(
+  instance.round({ smallestUnit: "second", roundingMode: "floor" }).epochNanoseconds,
+  -65_261_246_400_000_000_000n,  // -000099-12-15T12:00:00Z
+  "Rounding down is towards the Big Bang, not the epoch or 1 BCE (roundingMode floor)"
+);
+assert.sameValue(
+  instance.round({ smallestUnit: "second", roundingMode: "trunc" }).epochNanoseconds,
+  -65_261_246_400_000_000_000n,  // -000099-12-15T12:00:00Z
+  "Rounding down is towards the Big Bang, not the epoch or 1 BCE (roundingMode trunc)"
+);
+assert.sameValue(
+  instance.round({ smallestUnit: "second", roundingMode: "ceil" }).epochNanoseconds,
+  -65_261_246_399_000_000_000n,  // -000099-12-15T12:00:01Z
+  "Rounding up is away from the Big Bang, not the epoch or 1 BCE (roundingMode ceil)"
+);
+assert.sameValue(
+  instance.round({ smallestUnit: "second", roundingMode: "halfExpand" }).epochNanoseconds,
+  -65_261_246_399_000_000_000n,  // -000099-12-15T12:00:01Z
+  "Rounding up is away from the Big Bang, not the epoch or 1 BCE (roundingMode halfExpand)"
+);

test/built-ins/Temporal/ZonedDateTime/prototype/toString/rounding-direction.js

@@ -0,0 +1,30 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.tostring
+description: Rounding down is towards the Big Bang, not the epoch or 1 BCE
+features: [Temporal]
+---*/
+
+const instance = new Temporal.ZonedDateTime(-65_261_246_399_500_000_000n, "UTC");  // -000099-12-15T12:00:00.5Z
+assert.sameValue(
+  instance.toString({ smallestUnit: "second", roundingMode: "floor" }),
+  "-000099-12-15T12:00:00+00:00[UTC]",
+  "Rounding down is towards the Big Bang, not the epoch or 1 BCE"
+);
+assert.sameValue(
+  instance.toString({ smallestUnit: "second", roundingMode: "trunc" }),
+  "-000099-12-15T12:00:00+00:00[UTC]",
+  "Rounding down is towards the Big Bang, not the epoch or 1 BCE (roundingMode trunc)"
+);
+assert.sameValue(
+  instance.toString({ smallestUnit: "second", roundingMode: "ceil" }),
+  "-000099-12-15T12:00:01+00:00[UTC]",
+  "Rounding up is away from the Big Bang, not the epoch or 1 BCE (roundingMode ceil)"
+);
+assert.sameValue(
+  instance.toString({ smallestUnit: "second", roundingMode: "halfExpand" }),
+  "-000099-12-15T12:00:01+00:00[UTC]",
+  "Rounding up is away from the Big Bang, not the epoch or 1 BCE (roundingMode halfExpand)"
+);