Summary:
1. The buckifier script assume each test "foo" comes with a .cc file of the same name (i.e. foo.cc). Update cassandra tests to follow this pattern so that the buckifier script can recognize them.
2. add blob_db_test
Closes https://github.com/facebook/rocksdb/pull/2506

Differential Revision: D5331517

Pulled By: yiwu-arbug

fbshipit-source-id: 86f3eba471fc621186ab44cbd073b6162cde8e57