test: deflake test-fs-promises-watch-iterator

Add a delay before writing the files to ensure that the watcher receives
the notifications.

Fixes: #60051
Refs: #52601
PR-URL: #60060
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

Author: lpinca

test/parallel/test-fs-promises-watch-iterator.js

@@ -34,10 +34,12 @@ class WatchTestCase {
     }
   }
   async writeFiles() {
+    // Do the write with a delay to ensure that the OS is ready to notify us.
+    await setTimeout(common.platformTimeout(100));
+
     for (const fileName of [...this.files]) {
       await writeFile(this.filePath(fileName), Date.now() + fileName.repeat(1e4));
     }
-    await setTimeout(common.platformTimeout(100));
   }
 }