test: add coverage for dgram _createSocketHandle()

This commit adds code coverage to _createSocketHandle(), which
the cluster module uses to create dgram sockets.

PR-URL: #11291
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-dgram-create-socket-handle.js

@@ -0,0 +1,39 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const dgram = require('dgram');
+const UDP = process.binding('udp_wrap').UDP;
+const _createSocketHandle = dgram._createSocketHandle;
+
+// Throws if an "existing fd" is passed in.
+assert.throws(() => {
+  _createSocketHandle(common.localhostIPv4, 0, 'udp4', 42);
+}, /^AssertionError: false == true$/);
+
+{
+  // Create a handle that is not bound.
+  const handle = _createSocketHandle(null, null, 'udp4');
+
+  assert(handle instanceof UDP);
+  assert.strictEqual(typeof handle.fd, 'number');
+  assert(handle.fd < 0);
+}
+
+{
+  // Create a bound handle.
+  const handle = _createSocketHandle(common.localhostIPv4, 0, 'udp4');
+
+  assert(handle instanceof UDP);
+  assert.strictEqual(typeof handle.fd, 'number');
+
+  if (!common.isWindows)
+    assert(handle.fd > 0);
+}
+
+{
+  // Return an error if binding fails.
+  const err = _createSocketHandle('localhost', 0, 'udp4');
+
+  assert.strictEqual(typeof err, 'number');
+  assert(err < 0);
+}