workers: add test for messagePort.onmessage

PR-URL: https://github.com/nodejs/node/pull/21510
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-worker-onmessage.js

@@ -0,0 +1,18 @@
+// Flags: --experimental-worker
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const { Worker, isMainThread, parentPort } = require('worker_threads');
+
+if (isMainThread) {
+  const w = new Worker(__filename);
+  w.on('message', common.mustCall((message) => {
+    assert.strictEqual(message, 4);
+    w.terminate();
+  }));
+  w.postMessage(2);
+} else {
+  parentPort.onmessage = common.mustCall((message) => {
+    parentPort.postMessage(message * 2);
+  });
+}