Work around a flake in the Mac OS watcher test on the bots.

Issues 14373 and 14793 were tag-teaming to cause no_subscription_test
to fail for the Mac OS watcher occasionally.

This also marks directory_watcher/mac_os_test as timing out, apparently due to
issue 14943.

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//59143006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@30174 260f80e4-7a28-3924-810f-c04153c831b5