Turn on Timecop safe mode

Recently we've been burned by people leaving out Timecop.returnin their specs, which can lead to all sorts of subtle errors in other specs (e.g. gitlab-org/gitlab-ee#2362). We should strongly consider turning on safe mode to avoid this problem in the first place: https://github.com/travisjeffery/timecop#timecopsafe_mode

We could also consider checking the state of Timecop after each file is finished.