test: fix timeouts when running worker tests with --worker

These tests have been added after the original workers PR and
time out when run inside a worker by themselves.
This is needed for enabling `--worker` tests in our regular CI.

Refs: nodejs/build#1318

PR-URL: #21791
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Refael Ackermann <refack@gmail.com>

test/parallel/test-worker-exit-code.js

@@ -7,9 +7,11 @@ const common = require('../common');
 
 const assert = require('assert');
 const worker = require('worker_threads');
-const { Worker, isMainThread, parentPort } = worker;
+const { Worker, parentPort } = worker;
 
-if (isMainThread) {
+// Do not use isMainThread so that this test itself can be run inside a Worker.
+if (!process.env.HAS_STARTED_WORKER) {
+  process.env.HAS_STARTED_WORKER = 1;
   parent();
 } else {
   if (!parentPort) {

test/parallel/test-worker-onmessage.js

@@ -2,9 +2,11 @@
 'use strict';
 const common = require('../common');
 const assert = require('assert');
-const { Worker, isMainThread, parentPort } = require('worker_threads');
+const { Worker, parentPort } = require('worker_threads');
 
-if (isMainThread) {
+// Do not use isMainThread so that this test itself can be run inside a Worker.
+if (!process.env.HAS_STARTED_WORKER) {
+  process.env.HAS_STARTED_WORKER = 1;
   const w = new Worker(__filename);
   w.on('message', common.mustCall((message) => {
     assert.strictEqual(message, 4);