Summary:
Basic test cases:

- Manifest is lost or corrupt
- Manifest refers to too many or too few SST files
- SST file is corrupt
- Unflushed data is present when RepairDB is called

Depends on D55065 for its CreateFile() function in file_utils

Test Plan: Ran the tests.

Reviewers: IslamAbdelRahman, yhchiang, yoshinorim, sdong

Reviewed By: sdong

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D55485