Add a test for GridPaper (#4754)

Previously this widget was completely untested.

packages/flutter/test/widget/grid_paper_test.dart

+// Copyright 2015 The Chromium 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 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter/widgets.dart';
+
+void main() {
+  testWidgets('GridPaper control test', (WidgetTester tester) async {
+    await tester.pumpWidget(new GridPaper());
+    List<Layer> layers1 = tester.layers;
+    await tester.pumpWidget(new GridPaper());
+    List<Layer> layers2 = tester.layers;
+    expect(layers1, equals(layers2));
+  });
+}