Add tests for navigator_state.restorable_push.0.dart (#157667)

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

It adds a test for
- `examples/api/lib/widgets/navigator/navigator_state.restorable_push.0.dart`

examples/api/lib/widgets/navigator/navigator_state.restorable_push.0.dart

@@ -13,8 +13,12 @@ class RestorablePushExampleApp extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return const MaterialApp(
-      home: RestorablePushExample(),
+    return const RootRestorationScope(
+      restorationId: 'app',
+      child: MaterialApp(
+        restorationScopeId: 'app',
+        home: RestorablePushExample(),
+      ),
     );
   }
 }

examples/api/test/widgets/navigator/navigator_state.restorable_push.0_test.dart

@@ -0,0 +1,52 @@
+// 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/navigator/navigator_state.restorable_push.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('It pushes a restorable route and pops it', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.RestorablePushExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'Sample Code'), findsOne);
+    expect(find.byType(BackButton), findsNothing);
+
+    await tester.tap(find.widgetWithIcon(FloatingActionButton, Icons.add));
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.byType(BackButton));
+    await tester.pumpAndSettle();
+
+    expect(find.byType(BackButton), findsNothing);
+  });
+
+  testWidgets('It pushes a restorable route and restores it', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.RestorablePushExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'Sample Code'), findsOne);
+    expect(find.byType(BackButton), findsNothing);
+
+    await tester.tap(find.widgetWithIcon(FloatingActionButton, Icons.add));
+    await tester.pumpAndSettle();
+
+    await tester.restartAndRestore();
+
+    expect(find.byType(BackButton), findsOne);
+
+    final TestRestorationData data = await tester.getRestorationData();
+
+    await tester.tap(find.byType(BackButton));
+    await tester.pumpAndSettle();
+
+    expect(find.byType(BackButton), findsNothing);
+
+    await tester.restoreFrom(data);
+    expect(find.byType(BackButton), findsOne);
+  });
+}