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

samples/swarm/test/swarm_ui_lib/observable/observable_value_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;
 
 testObservableValue() {
   test('ObservableValue', () {
     final value = new ObservableValue<String>('initial');
     expect(value.value, equals('initial'));
@@ skipping 24 matching lines @@
     bool called = false;
     value.addChangeListener((summary) { called = true; });
 
     // Set it to the same value.
     value.value = 'foo';
 
     // Should not have gotten an event.
     expect(called, isFalse);
   });
 }