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

testing_app/integration_test/driver.dart

@@ -0,0 +1,23 @@
+// 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:integration_test/integration_test_driver.dart';
+
+Future<void> main() {
+  return integrationDriver(
+    responseDataCallback: (data) async {
+      // If the tests reported any data, save it to the disk.
+      if (data != null) {
+        for (var entry in data.entries) {
+          print('Writing ${entry.key} to the disk.');
+          // Default storage destination is the 'build' directory.
+          await writeResponseData(
+            entry.value as Map<String, dynamic>,
+            testOutputFilename: entry.key,
+          );
+        }
+      }
+    },
+  );
+}