Limit the time util/task_test can run.

There was a run of another pull request on Travis that got stuck:

https://travis-ci.org/google/certificate-transparency/builds/44202920

Re-running it unclogged it, but that's just bad news, it should never happen. So this should both help fail quicker if we're stuck on a ```Notification```, and tell us which one was stuck (to help debugging).