Delete rocksdb dir after crashtest

Summary:
We should clean up after ourselves. Last crashtest failed because the disk on the cibuild machine was full.

Also, db_crashtest is supposed to run the test on the same database in every iteration. This isn't the case currently, because every run creates a new tmp directory. It's fixed with this diff.

Test Plan: ran it

Reviewers: ljin

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17073