test_runner: wait for stderr and stdout to complete

PR-URL: #43666
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

lib/internal/main/test_runner.js

@@ -7,13 +7,16 @@ const {
   ArrayPrototypeSlice,
   ArrayPrototypeSort,
   Promise,
+  PromiseAll,
+  SafeArrayIterator,
   SafeSet,
 } = primordials;
 const {
   prepareMainThreadExecution,
 } = require('internal/bootstrap/pre_execution');
 const { spawn } = require('child_process');
 const { readdirSync, statSync } = require('fs');
+const { finished } = require('internal/streams/end-of-stream');
 const console = require('internal/console/global');
 const {
   codes: {
@@ -126,9 +129,10 @@ function runTestFile(path) {
         stderr += chunk;
       });
 
-      child.once('exit', (code, signal) => {
+      child.once('exit', async (code, signal) => {
         if (code !== 0 || signal !== null) {
           if (!err) {
+            await PromiseAll(new SafeArrayIterator([finished(child.stderr), finished(child.stdout)]));
             err = new ERR_TEST_FAILURE('test failed', kSubtestsFailed);
             err.exitCode = code;
             err.signal = signal;