Add a unit test for behavior when merge operator and compaction filter co-exist.

Summary: Add a unit test in db_test to verify the behavior when both of merge operator and compaction filter apply to a key when merging.

Test Plan: Run the new test

Reviewers: ljin, yhchiang, rven, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28455