Move test's filesystem cleanup into TH::clearFilesystem, call from clearEnv

Closes #1243

I suggest that clearing the filesystem is in no way different than clearing mysql, mongodb, memcached and so on..
So moving this code into `clearEnv` makes sense, and will allow us to get rid of that *shutdown function*.
Test cases should clean up after themselves. If they don't I assume it's a bug, and they should call `clearEnv` or do it themselves.

@fauvel @tomaszdurka what do you think?