[material_ui] Enable popup_menu_test (#12018)

Port over `popup_menu_test` from flutter/flutter#184279

Work towards flutter/flutter#182636 and flutter/flutter#188395

## Pre-Review Checklist

**Note**: The Flutter team is currently trialing the use of [Gemini Code Assist for GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code). Comments from the `gemini-code-assist` bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: elliette

…rily_disabled_tests/popup_menu_test.dart …es/material_ui/test/popup_menu_test.dartpackages/material_ui/temporarily_disabled_tests/popup_menu_test.dart renamed to packages/material_ui/test/popup_menu_test.dart packages/material_ui/temporarily_disabled_tests/popup_menu_test.dart renamed to packages/material_ui/test/popup_menu_test.dart

@@ -2,18 +2,16 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-@Skip(
-  'This file is skipped due to a cross-import that needs to be fixed. Tracked in https://github.com/flutter/flutter/issues/177028.',
-)
+import 'dart:async';
 import 'dart:ui';
 
 import 'package:flutter/foundation.dart';
-import 'package:material_ui/material_ui.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:material_ui/material_ui.dart';
 
-import '../widgets/feedback_tester.dart';
-import '../widgets/semantics_tester.dart';
+import 'feedback_tester.dart';
+import 'semantics_tester.dart';
 
 void main() {
   testWidgets('Navigator.push works within a PopupMenuButton', (WidgetTester tester) async {
@@ -3725,11 +3723,13 @@ void main() {
     final BuildContext context = tester.element(find.text('Go'));
     const exampleSetting = RouteSettings(name: 'simple');
 
-    showMenu<void>(
-      context: context,
-      position: RelativeRect.fill,
-      items: const <PopupMenuItem<void>>[PopupMenuItem<void>(child: Text('foo'))],
-      routeSettings: exampleSetting,
+    unawaited(
+      showMenu<void>(
+        context: context,
+        position: RelativeRect.fill,
+        items: const <PopupMenuItem<void>>[PopupMenuItem<void>(child: Text('foo'))],
+        routeSettings: exampleSetting,
+      ),
     );
 
     await tester.pumpAndSettle();