Only run missing_tick_test for 2 seconds, before timeing out.

pkg/unittest/test/missing_tick_test.dart

 // Copyright (c) 2013, 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.
 
 import 'package:unittest/unittest.dart';
 
 // TODO(gram): Convert to a shouldFail passing test.
 main() {
+  SimpleConfiguration config = unittestConfiguration;
+  config.timeout = const Duration(seconds: 2);
   group('Broken', () {
     test('test that should time out', () {
       expectAsync0(() {});
     });
   });
 }