test: improve worker_threads coverage

PR-URL: nodejs#41818
Refs: https://coverage.nodejs.org/coverage-7123a00b03a90862/lib/internal/worker.js.html#L412
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-worker-environmentdata.js

@@ -1,4 +1,5 @@
 'use strict';
+// Flags: --expose-internals
 
 require('../common');
 const {
@@ -8,6 +9,8 @@ const {
   threadId,
 } = require('worker_threads');
 
+const { assignEnvironmentData } = require('internal/worker');
+
 const {
   deepStrictEqual,
   strictEqual,
@@ -26,6 +29,8 @@ if (!process.env.HAS_STARTED_WORKER) {
   strictEqual(getEnvironmentData('foo'), 'bar');
   deepStrictEqual(getEnvironmentData('hello'), { value: 'world' });
   strictEqual(getEnvironmentData(1), undefined);
+  assignEnvironmentData(undefined); // It won't setup any key.
+  strictEqual(getEnvironmentData(undefined), undefined);
 
   // Recurse to make sure the environment data is inherited
   if (threadId <= 2)

test/parallel/test-worker-heap-snapshot.js

@@ -0,0 +1,16 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const { Worker } = require('worker_threads');
+const { once } = require('events');
+
+// Ensure that worker.getHeapSnapshot() returns a valid JSON
+(async () => {
+  const worker = new Worker('setInterval(() => {}, 1000);', { eval: true });
+  await once(worker, 'online');
+  const stream = await worker.getHeapSnapshot();
+  assert.ok(JSON.parse(stream.read()));
+
+  await worker.terminate();
+})().then(common.mustCall());