[testing_app] Migrate the sample to the integration_test package (#633)

testing_app/integration_test/state_mgmt_test.dart

@@ -0,0 +1,50 @@
+// Copyright 2020 The Flutter team. 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_test/flutter_test.dart';
+import 'package:integration_test/integration_test.dart';
+import 'package:provider/provider.dart';
+import 'package:testing_app/models/favorites.dart';
+import 'package:testing_app/screens/favorites.dart';
+
+Favorites favoritesList;
+
+Widget createFavoritesScreen() => ChangeNotifierProvider<Favorites>(
+      create: (context) {
+        favoritesList = Favorites();
+        return favoritesList;
+      },
+      child: MaterialApp(
+        home: FavoritesPage(),
+      ),
+    );
+
+void main() {
+  group('Testing App State Management Tests', () {
+    IntegrationTestWidgetsFlutterBinding.ensureInitialized();
+
+    testWidgets('Verifying add method', (tester) async {
+      await tester.pumpWidget(createFavoritesScreen());
+
+      //  Add an item to the list.
+      favoritesList.add(30);
+      await tester.pumpAndSettle();
+
+      // Check if the new item appears in the list.
+      expect(find.text('Item 30'), findsOneWidget);
+    });
+
+    testWidgets('Verifying remove method', (tester) async {
+      await tester.pumpWidget(createFavoritesScreen());
+
+      // Remove an item from the list.
+      favoritesList.remove(30);
+      await tester.pumpAndSettle();
+
+      // Verify if it disappears.
+      expect(find.text('Item 30'), findsNothing);
+    });
+  });
+}