Add a unit test to verify compaction filter context

Summary: Add unit tests to make sure CompactionFilterContext::is_manual_compaction_ and CompactionFilterContext::is_full_compaction_ are set correctly.

Test Plan: run the new tests.

Reviewers: haobo, igor, dhruba, yhchiang, ljin

Reviewed By: haobo

CC: nkg-, leveldb

Differential Revision: https://reviews.facebook.net/D17067