Add test for restorable_route_future.0.dart (#157708)

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

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

examples/api/test/widgets/navigator/restorable_route_future.0_test.dart

@@ -0,0 +1,80 @@
+// 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/restorable_route_future.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.RestorableRouteFutureExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'RestorableRouteFuture Example'), findsOne);
+    expect(find.byType(BackButton), findsNothing);
+
+    expect(find.text('Last count: 0'), findsOne);
+
+    await tester.tap(find.widgetWithText(ElevatedButton, 'Open Counter'));
+    await tester.pumpAndSettle();
+
+    expect(find.widgetWithText(AppBar, 'Awesome Counter'), findsOne);
+    expect(find.text('Count: 0'), findsOne);
+
+    await tester.tap(find.widgetWithIcon(FloatingActionButton, Icons.add));
+    await tester.pump();
+
+    expect(find.text('Count: 1'), findsOne);
+
+    await tester.tap(find.byType(BackButton));
+    await tester.pumpAndSettle();
+
+    expect(find.widgetWithText(AppBar, 'RestorableRouteFuture Example'), findsOne);
+    expect(find.text('Last count: 1'), findsOne);
+  });
+
+  testWidgets('It pushes a restorable route and restores it', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.RestorableRouteFutureExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'RestorableRouteFuture Example'), findsOne);
+    expect(find.byType(BackButton), findsNothing);
+
+    expect(find.text('Last count: 0'), findsOne);
+
+    await tester.tap(find.widgetWithText(ElevatedButton, 'Open Counter'));
+    await tester.pumpAndSettle();
+
+    expect(find.widgetWithText(AppBar, 'Awesome Counter'), findsOne);
+    expect(find.text('Count: 0'), findsOne);
+
+    await tester.tap(find.widgetWithIcon(FloatingActionButton, Icons.add));
+    await tester.pump();
+
+    expect(find.text('Count: 1'), findsOne);
+
+    await tester.restartAndRestore();
+    expect(find.byType(BackButton), findsOne);
+
+    expect(find.text('Count: 1'), findsOne);
+
+    final TestRestorationData data = await tester.getRestorationData();
+
+    await tester.tap(find.byType(BackButton));
+    await tester.pumpAndSettle();
+
+    expect(find.byType(BackButton), findsNothing);
+
+    expect(find.widgetWithText(AppBar, 'RestorableRouteFuture Example'), findsOne);
+    expect(find.text('Last count: 1'), findsOne);
+
+    await tester.restoreFrom(data);
+
+    expect(find.widgetWithText(AppBar, 'Awesome Counter'), findsOne);
+    expect(find.byType(BackButton), findsOne);
+    expect(find.text('Count: 1'), findsOne);
+  });
+}