Add an e2e (host-independent) test demo to testing_app (#515)

testing_app/test_driver/e2e_test.dart

@@ -0,0 +1,21 @@
+// 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 'dart:async';
+
+import 'package:e2e/e2e_driver.dart' as driver;
+
+Future<void> main() => driver.e2eDriver(responseDataCallback: (data) async {
+      await driver.writeResponseData(
+        data['scrolling'] as Map<String, dynamic>,
+        // This result is saved to `build/scrolling.json`.
+        testOutputFilename: 'scrolling',
+      );
+
+      await driver.writeResponseData(
+        data['favorites_operations'] as Map<String, dynamic>,
+        // This result is saved to `build/favorites_operations.json`.
+        testOutputFilename: 'favorites_operations',
+      );
+    });