Only run missing_tick_test for 2 seconds, before timeing out.

BUG=
R=ahe@google.com

Review URL: https://codereview.chromium.org//62363002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/unittest@30035 260f80e4-7a28-3924-810f-c04153c831b5

test/missing_tick_test.dart

@@ -6,6 +6,8 @@ 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(() {});