Fix regression test

Summary:
https://github.com/facebook/rocksdb/commit/388d2054c7077d58ea56e08092382d904bec71c7
added extra line to db_bench output, breaking regression tests. This diff makes it more robust and fixes the issue

Test Plan: ran it

Reviewers: ljin, sdong

Reviewed By: sdong

Subscribers: sdong, leveldb

Differential Revision: https://reviews.facebook.net/D18897