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()`');
 
-functionrestart(file,content=readFileSync(file)){
-writeFileSync(file,content);
-consttimer=setInterval(()=>writeFileSync(file,content),common.platformTimeout(250));
+functionrestart(file){
+appendFileSync(file,'\n');
+consttimer=setInterval(()=>appendFileSync(file,'\n'),common.platformTimeout(250));
 return()=>clearInterval(timer);
 }
 
@@ -26,6 +26,12 @@ function createTmpFile(content = 'console.log(\'running\');', ext = '.js', basen
 returnfile;
 }
 
+functioncreateTmpDir(){
+constdir=path.join(tmpdir.path,`${tmpFiles++}`);
+mkdirSync(dir);
+returndir;
+}
+
 asyncfunctionrunWriteSucceed({
  file,
  watchedFile,
@@ -78,10 +84,10 @@ async function runWriteSucceed({
 }
 
 tmpdir.refresh();
-constdir=tmpdir.path;
 
 describe('watch mode',{concurrency: !process.env.TEST_PARALLEL,timeout: 60_000},()=>{
 it('should watch changes to worker - cjs',async()=>{
+constdir=createTmpDir();
 constworker=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()=>{
+constdir=createTmpDir();
 constdep=path.join(dir,'dep.js');
 constworker=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()=>{
+constdir=createTmpDir();
 constsubDep=path.join(dir,'sub-dep.js');
 constdep=path.join(dir,'dep.js');
 constworker=path.join(dir,'worker.js');
@@ -181,6 +189,7 @@ const w = new Worker(${JSON.stringify(worker)});
 });
 
 it('should watch changes to worker - esm',async()=>{
+constdir=createTmpDir();
 constworker=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()=>{
+constdir=createTmpDir();
 constdep=path.join(dir,'dep.mjs');
 constworker=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()=>{
+constdir=createTmpDir();
 constsubDep=path.join(dir,'sub-dep.mjs');
 constdep=path.join(dir,'dep.mjs');
 constworker=path.join(dir,'worker.mjs');