📄 Add license block to test/widgets_test.dart

Author: ValentinVignal

packages/google_sign_in/google_sign_in/test/widgets_test.dart

@@ -1,3 +1,7 @@
+// Copyright 2013 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 'dart:async';
 import 'dart:convert';
 import 'dart:io';