test: stabilize test-vfs-watch-directory

The first block used persistent: false plus setTimeout to trigger the
write. The watcher's poll timer was unref'd, so on slow runners the
write timer could fire before the first poll and the change event
would be missed. Use the same await-once pattern as the other blocks
in the file with a content-length change so the size-based stat-change
detector always fires.

Assisted-by: Claude-Opus4.7
Signed-off-by: Matteo Collina <hello@matteocollina.com>

Author: mcollina

test/parallel/test-vfs-watch-directory.js

@@ -11,24 +11,20 @@ const assert = require('assert');
 const { once } = require('events');
 const vfs = require('node:vfs');
 
-// Modifying a child file of a watched directory must emit a change event.
-{
-  const myVfs = vfs.create();
-  myVfs.mkdirSync('/parent', { recursive: true });
-  myVfs.writeFileSync('/parent/file.txt', 'x');
-
-  const watcher = myVfs.watch('/parent', {
-    interval: 50,
-    persistent: false,
-  }, common.mustCall((eventType, filename) => {
+(async () => {
+  // Modifying a child file emits a change event.
+  {
+    const myVfs = vfs.create();
+    myVfs.mkdirSync('/parent', { recursive: true });
+    myVfs.writeFileSync('/parent/file.txt', 'x');
+    const watcher = myVfs.watch('/parent', { interval: 25 });
+    const changed = once(watcher, 'change');
+    myVfs.writeFileSync('/parent/file.txt', 'longer-content-changed');
+    const [, filename] = await changed;
     assert.strictEqual(filename, 'file.txt');
     watcher.close();
-  }));
-
-  setTimeout(() => myVfs.writeFileSync('/parent/file.txt', 'y'), 100);
-}
+  }
 
-(async () => {
   // Non-recursive directory watch: file creation
   {
     const myVfs = vfs.create();