Add test for restoration_mixin.0.dart (#157709)

Contributes to https://github.com/flutter/flutter/issues/130459

It adds a test for
- `examples/api/lib/widgets/restoration/restoration_mixin.0.dart`

dev/bots/check_code_samples.dart

@@ -328,6 +328,5 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/notification_listener/notification.0_test.dart',
   'examples/api/test/widgets/overscroll_indicator/glowing_overscroll_indicator.1_test.dart',
   'examples/api/test/widgets/overscroll_indicator/glowing_overscroll_indicator.0_test.dart',
-  'examples/api/test/widgets/restoration/restoration_mixin.0_test.dart',
   'examples/api/test/widgets/focus_scope/focus_scope.0_test.dart',
 };

examples/api/test/widgets/restoration/restoration_mixin.0_test.dart

@@ -0,0 +1,37 @@
+// 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/material.dart';
+import 'package:flutter_api_samples/widgets/restoration/restoration_mixin.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('The state of the counter can be restored', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.RestorationExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'Restorable Counter'), findsOne);
+    expect(find.text('You have pushed the button this many times:'), findsOne);
+    expect(find.text('0'), findsOne);
+
+    await tester.tap(find.widgetWithIcon(FloatingActionButton, Icons.add));
+    await tester.pump();
+    expect(find.text('1'), findsOne);
+
+    await tester.restartAndRestore();
+
+    expect(find.text('1'), findsOne);
+
+    final TestRestorationData data = await tester.getRestorationData();
+
+    await tester.tap(find.widgetWithIcon(FloatingActionButton, Icons.add));
+    await tester.pump();
+    expect(find.text('2'), findsOne);
+
+    await tester.restoreFrom(data);
+
+    expect(find.text('1'), findsOne);
+  });
+}