test: wait for native repository watcher readiness

tests/repository-monitor.test.mjs

@@ -83,6 +83,9 @@ test('a watched repository is read on filesystem activity instead of on every in
   await monitor.tick();
   assert.equal(reads.length, 1);
 
+  // fs.watch can be constructed before the underlying native watch has
+  // finished subscribing (notably on Linux overlay filesystems in CI).
+  await new Promise((resolve) => setTimeout(resolve, 150));
   revision = 2;
   await writeFile(path.join(root, 'feature.txt'), 'changed\n');
   // The watcher debounce and the per-repository cooldown both apply here.