Summary:
Re-enable blob_db_test with some update:
* Commented out delay at the end of GC tests. Will update the logic later with sync point to properly trigger GC.
* Added some helper functions.

Also update make files to include blob_dump tool.
Closes https://github.com/facebook/rocksdb/pull/2375

Differential Revision: D5133793

Pulled By: yiwu-arbug

fbshipit-source-id: 95470b26d0c1f9592ba4b7637e027fdd263f425c