Add a test in compaction_picker_test to test the max score

Summary: Add a new unit test in compaction_picker_test to make sure level-based compaction to pick up the level with the largest score.

Test Plan: Run the new test

Reviewers: ljin, yhchiang, rven, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D27933