test: add tests for end event of stream.Duplex

Added tests to check the stream will automatically end the writable side
when readable side ends when allowHalfOpen option is false.

PR-URL: #21325
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>

test/parallel/test-stream-duplex-end.js

@@ -0,0 +1,41 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const Duplex = require('stream').Duplex;
+
+{
+  const stream = new Duplex({
+    read() {}
+  });
+  assert.strictEqual(stream.allowHalfOpen, true);
+  stream.on('finish', common.mustNotCall());
+  assert.strictEqual(stream.listenerCount('end'), 0);
+  stream.resume();
+  stream.push(null);
+}
+
+{
+  const stream = new Duplex({
+    read() {},
+    allowHalfOpen: false
+  });
+  assert.strictEqual(stream.allowHalfOpen, false);
+  stream.on('finish', common.mustCall());
+  assert.strictEqual(stream.listenerCount('end'), 1);
+  stream.resume();
+  stream.push(null);
+}
+
+{
+  const stream = new Duplex({
+    read() {},
+    allowHalfOpen: false
+  });
+  assert.strictEqual(stream.allowHalfOpen, false);
+  stream._writableState.ended = true;
+  stream.on('finish', common.mustNotCall());
+  assert.strictEqual(stream.listenerCount('end'), 1);
+  stream.resume();
+  stream.push(null);
+}