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

samples/swarm/test/swarm_ui_lib/observable/abstract_observable_tests.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 observable_tests;
 
 testAbstractObservable() {
   group('addChangeListener()', () {
     test('adding the same listener twice returns false the second time', () {
       final target = new AbstractObservable();
@@ skipping 55 matching lines @@
     validateUpdate(res.events[0], target, 'pM', null, 10, 11);
     res = null;
 
     target.recordPropertyUpdate('pL', '11', '13');
 
     expect(res, isNotNull);
     expect(res.events, hasLength(1));
     validateUpdate(res.events[0], target, 'pL', null, '11', '13');
   });
 }