Unit tests of RocksDB

Although FDB end-to-end simulation test is good at finding bugs, it is hard to narrow down the problem.

Considering RocksDB is a promising storage engine for FDB, we should consider adding unit tests to check the correctness of RocksDB.

That will also help us to get help from RocksDB community to fix any RocksDB bugs we found.

I think Redwood's unit tests can be a reference point. Those unit tests can be found by `find ./tests -name "Redwood*.txt"`