Summary:
I'm going to add more DB tests for statistics as currently we have very few. I started a file dedicated to this purpose and moved the existing stats-specific tests there.
Closes https://github.com/facebook/rocksdb/pull/2211

Differential Revision: D4951558

Pulled By: ajkr

fbshipit-source-id: 05d11c35079c40ecabdfd2cf5556ccb761f694a4