Fix unit tests errors

Summary: Those were introduced with https://github.com/facebook/rocksdb/commit/2fb1fea30fd027bbd824a26b682d04d91a8661dc because the flushing behavior changed when max_background_flushes is > 0.

Test Plan: make check

Reviewers: ljin, yhchiang, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23577