test: deflake watch mode worker test

Trigger watch restarts by appending whitespace instead of rewriting
watched modules. This avoids transient empty or partial ESM dependency
contents while the restarted worker is loading.

Use a separate temporary directory for each subtest so concurrent
subtests do not share worker and dependency file names.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: openai:gpt-5.5
PR-URL: #63384
Refs: https://github.com/nodejs/reliability/blob/main/reports/2026-05-17.md#jstest-failure
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

Author: trivikr

test/sequential/test-watch-mode-worker.mjs

@@ -5,17 +5,17 @@ import path from 'node:path';
 import { execPath } from 'node:process';
 import { describe, it } from 'node:test';
 import { spawn } from 'node:child_process';
-import { writeFileSync, readFileSync } from 'node:fs';
+import { appendFileSync, mkdirSync, writeFileSync } from 'node:fs';
 import { inspect } from 'node:util';
 import { pathToFileURL } from 'node:url';
 import { createInterface } from 'node:readline';
 
 if (common.isIBMi)
   common.skip('IBMi does not support `fs.watch()`');
 
-function restart(file, content = readFileSync(file)) {
-  writeFileSync(file, content);
-  const timer = setInterval(() => writeFileSync(file, content), common.platformTimeout(250));
+function restart(file) {
+  appendFileSync(file, '\n');
+  const timer = setInterval(() => appendFileSync(file, '\n'), common.platformTimeout(250));
   return () => clearInterval(timer);
 }
 
@@ -26,6 +26,12 @@ function createTmpFile(content = 'console.log(\'running\');', ext = '.js', basen
   return file;
 }
 
+function createTmpDir() {
+  const dir = path.join(tmpdir.path, `${tmpFiles++}`);
+  mkdirSync(dir);
+  return dir;
+}
+
 async function runWriteSucceed({
   file,
   watchedFile,
@@ -78,10 +84,10 @@ async function runWriteSucceed({
 }
 
 tmpdir.refresh();
-const dir = tmpdir.path;
 
 describe('watch mode', { concurrency: !process.env.TEST_PARALLEL, timeout: 60_000 }, () => {
   it('should watch changes to worker - cjs', async () => {
+    const dir = createTmpDir();
     const worker = path.join(dir, 'worker.js');
 
     writeFileSync(worker, `
@@ -109,6 +115,7 @@ const w = new Worker(${JSON.stringify(worker)});
   });
 
   it('should watch changes to worker dependencies - cjs', async () => {
+    const dir = createTmpDir();
     const dep = path.join(dir, 'dep.js');
     const worker = path.join(dir, 'worker.js');
 
@@ -142,6 +149,7 @@ const w = new Worker(${JSON.stringify(worker)});
   });
 
   it('should watch changes to nested worker dependencies - cjs', async () => {
+    const dir = createTmpDir();
     const subDep = path.join(dir, 'sub-dep.js');
     const dep = path.join(dir, 'dep.js');
     const worker = path.join(dir, 'worker.js');
@@ -181,6 +189,7 @@ const w = new Worker(${JSON.stringify(worker)});
   });
 
   it('should watch changes to worker - esm', async () => {
+    const dir = createTmpDir();
     const worker = path.join(dir, 'worker.mjs');
 
     writeFileSync(worker, `
@@ -208,6 +217,7 @@ new Worker(new URL(${JSON.stringify(pathToFileURL(worker))}));
   });
 
   it('should watch changes to worker dependencies - esm', async () => {
+    const dir = createTmpDir();
     const dep = path.join(dir, 'dep.mjs');
     const worker = path.join(dir, 'worker.mjs');
 
@@ -241,6 +251,7 @@ new Worker(new URL(${JSON.stringify(pathToFileURL(worker))}));
   });
 
   it('should watch changes to nested worker dependencies - esm', async () => {
+    const dir = createTmpDir();
     const subDep = path.join(dir, 'sub-dep.mjs');
     const dep = path.join(dir, 'dep.mjs');
     const worker = path.join(dir, 'worker.mjs');