yaml_test: adapt around undefined behavior in float64->int64 casting

In TestJSONObjectToYAMLObject the "uint64 big" case accepts a float64
of size 2^63. The value is passed to jsonToYAMLValue() and the
float64 branch of the switch is entered. For values that do not
fit int64 the first cast to int64() is undefined behavior
in most languages and apparently in Golang:
http://c0x.coding-guidelines.com/6.3.1.4.pdf

This means the value of int64(float64(2^63)) can end up as either
-9223372036854775808 or 9223372036854775807.
From experimentation, it appears compiler optimization
determines the result.

The value is then casted back to float64 and matched against
the original float64, which may or may not pass. Depending of FPU
rounding mode for IEEE754-Doubles any input above 9223372036854775296
may get casted to 9223372036854775808.0.

Adapt the unit test for "uint64 big" to not feed a big float64 and enter
the problem cases. Instead pass the nearest rounded value of 2^63
as a uint64 and expect to receive the same result:
  bigUint64 = ((uint64(1) << 63) + 500) / 1000 * 1000

yaml_test.go

@@ -428,6 +428,7 @@ foo: baz
 }
 
 func TestJSONObjectToYAMLObject(t *testing.T) {
+	const bigUint64 = ((uint64(1) << 63) + 500) / 1000 * 1000
 	intOrInt64 := func(i64 int64) interface{} {
 		if i := int(i64); i64 == int64(i) {
 			return i
@@ -459,7 +460,7 @@ func TestJSONObjectToYAMLObject(t *testing.T) {
 				"map":                map[string]interface{}{"foo": "bar"},
 				"slice":              []interface{}{"foo", "bar"},
 				"string":             string("foo"),
-				"uint64 big":         float64(math.Pow(2, 63)),
+				"uint64 big":         bigUint64,
 			},
 			expected: yaml.MapSlice{
 				{Key: "nil slice"},
@@ -476,7 +477,7 @@ func TestJSONObjectToYAMLObject(t *testing.T) {
 				{Key: "map", Value: yaml.MapSlice{{Key: "foo", Value: "bar"}}},
 				{Key: "slice", Value: []interface{}{"foo", "bar"}},
 				{Key: "string", Value: string("foo")},
-				{Key: "uint64 big", Value: uint64(1) << 63},
+				{Key: "uint64 big", Value: bigUint64},
 			},
 		},
 	}