test: fix race condition in watch mode tests

The watch mode tests using runInBackground() had a race condition where
stdout "Failed running" message could arrive before stderr was fully
collected, causing assertions on stderr content to fail with empty
strings.

Fix by waiting for stderr 'data' event if stderr is empty when "Failed
running" is detected. This is an event-driven approach rather than
timing-based.

Refs: nodejs/reliability#1450

Author: mcollina

test/sequential/test-watch-mode-restart-esm-loading-error.mjs

@@ -42,14 +42,23 @@ function runInBackground({ args = [], options = {}, completed = 'Completed runni
           stdout = [];
           stderr = '';
         } else if (data.startsWith('Failed running')) {
-          if (shouldFail) {
-            future.resolve({ stderr, stdout });
+          const settle = () => {
+            if (shouldFail) {
+              future.resolve({ stderr, stdout });
+            } else {
+              future.reject({ stderr, stdout });
+            }
+            future = Promise.withResolvers();
+            stdout = [];
+            stderr = '';
+          };
+          // If stderr is empty, wait for it to receive data before settling.
+          // This handles the race condition where stdout arrives before stderr.
+          if (stderr === '') {
+            child.stderr.once('data', settle);
           } else {
-            future.reject({ stderr, stdout });
+            settle();
           }
-          future = Promise.withResolvers();
-          stdout = [];
-          stderr = '';
         }
       }
     });

test/sequential/test-watch-mode.mjs

@@ -54,14 +54,23 @@ function runInBackground({ args = [], options = {}, completed = 'Completed runni
           stdout = [];
           stderr = '';
         } else if (data.startsWith('Failed running')) {
-          if (shouldFail) {
-            future.resolve({ stderr, stdout });
+          const settle = () => {
+            if (shouldFail) {
+              future.resolve({ stderr, stdout });
+            } else {
+              future.reject({ stderr, stdout });
+            }
+            future = Promise.withResolvers();
+            stdout = [];
+            stderr = '';
+          };
+          // If stderr is empty, wait for it to receive data before settling.
+          // This handles the race condition where stdout arrives before stderr.
+          if (stderr === '') {
+            child.stderr.once('data', settle);
           } else {
-            future.reject({ stderr, stdout });
+            settle();
           }
-          future = Promise.withResolvers();
-          stdout = [];
-          stderr = '';
         }
       }
     });