Cover more tests with leak tracking. (#132806)

This PR replaces some `testWidgets` with `testWidgestsWithLeakTracking`.

packages/flutter/test/material/back_button_test.dart

@@ -8,7 +8,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:leak_tracker_flutter_testing/leak_tracker_flutter_testing.dart';
 
 void main() {
-  testWidgets('BackButton control test', (WidgetTester tester) async {
+  testWidgetsWithLeakTracking('BackButton control test', (WidgetTester tester) async {
     await tester.pumpWidget(
       MaterialApp(
         home: const Material(child: Text('Home')),