Adds espresso testing for add_to_app/android_fullscreen (#323)

add_to_app/flutter_module/test_driver/example.dart

@@ -0,0 +1,13 @@
+// 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_driver/driver_extension.dart';
+import 'package:flutter_module/main.dart' as app;
+
+// This alternate entrypoint is used for espresso testing. See
+// https://pub.dev/packages/espresso for details.
+void main() {
+  enableFlutterDriverExtension();
+  app.main();
+}