Disposes test restoration manager when accessed by bindings (#176519)

Fixes #169119

## Details

The error caused by drag is due to the reference to
`ServicesBinding.instance.restorationManager` in the
`ScrollableState.saveOffset` method.


https://github.com/flutter/flutter/blob/018897e3f12c5783fb025bcbd92515a69d4d5c32/packages/flutter/lib/src/widgets/scrollable.dart#L648-L656

The TestWidgetsFlutterBinding now only cleans up the
`restorationManager` when `reset` is called. Therefore, we need to find
an appropriate place to clean up the `restorationManager`.


https://github.com/flutter/flutter/blob/018897e3f12c5783fb025bcbd92515a69d4d5c32/packages/flutter_test/lib/src/binding.dart#L239-L263


https://github.com/flutter/flutter/blob/018897e3f12c5783fb025bcbd92515a69d4d5c32/packages/flutter_test/lib/src/widget_tester.dart#L188-L193

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

---------

Co-authored-by: Victor Sanni <victorsanniay@gmail.com>

Author: hgraceb

packages/flutter_test/lib/src/binding.dart

@@ -1067,6 +1067,8 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
         Container(key: UniqueKey(), child: _postTestMessage),
       ); // Unmount any remaining widgets.
       await pump();
+      _restorationManager?.dispose();
+      _restorationManager = null;
       if (registerTestTextInput) {
         _testTextInput.unregister();
       }

packages/flutter_test/test/widget_tester_leaks_free_test.dart

@@ -0,0 +1,29 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/services.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:leak_tracker_flutter_testing/leak_tracker_flutter_testing.dart';
+
+void main() {
+  LeakTesting.enable(); // Enable leak testing and use default collectedLeaksReporter.
+
+  // Regression test for https://github.com/flutter/flutter/issues/169119.
+  testWidgets('Does not leak if restorationManager is accessed', (WidgetTester tester) async {
+    int counterByWidgets = 0;
+    final RestorationManager managerByWidgets = WidgetsBinding.instance.restorationManager;
+    expect(managerByWidgets, isA<TestRestorationManager>());
+    managerByWidgets.addListener(() => counterByWidgets++);
+    managerByWidgets.notifyListeners();
+    expect(counterByWidgets, 1);
+
+    int counterByServices = 0;
+    final RestorationManager managerByServices = ServicesBinding.instance.restorationManager;
+    expect(managerByServices, isA<TestRestorationManager>());
+    managerByServices.addListener(() => counterByServices++);
+    managerByServices.notifyListeners();
+    expect(counterByServices, 1);
+  });
+}