Reorganize tests for samples so the test is discoverable from the sample

samples/swarm/test/swarm_ui_lib/layout/grid_layout_demo.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of layout_tests;
 
 /**
  * An app for testing the grid layout system.
  */
 
@@ skipping 108 matching lines @@
     window.console.log(sb.toString());
   });
 }
 
 void _appendMetrics(StringBuffer sb, Element node, [String indent = '']) {
   String id = node.id;
   num left = node.offsetLeft, top = node.offsetTop;
   num width = node.offsetWidth, height = node.offsetHeight;
   sb.write("${indent}'$id': [$left, $top, $width, $height],\n");
 }