Fix checkpoint_test directory setup/cleanup
Fix checkpoint_test directory setup/cleanup

Summary:
- Change directory name from "db_test" to "checkpoint_test". Previously it used the same directory as `db_test`
- Systematically cleanup snapshot and snapshot staging directories before each test. Previously a failed test run caused subsequent runs to fail, particularly when the first failure caused "snapshot.tmp" to not be cleaned up.
Closes https://github.com/facebook/rocksdb/pull/3351

Differential Revision: D6691015

Pulled By: ajkr

fbshipit-source-id: 4fc2ac2e21ff2617ea0e96297c5132b5f2eefd79

SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB