Summary:
GetTestDirectory implies a file system operation (it creates the
default test directory if missing), so it should be routed to
the FileSystem rather than the Env.
Also remove the GetTestDirectory implementation in the PosixEnv,
since it overrides GetTestDirectory in CompositeEnv making it
impossible to override with a custom FileSystem.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6896
Reviewed By: cheng-chang
Differential Revision: D21868984
Pulled By: ajkr
fbshipit-source-id: e79bfef758d06dacef727c54b96abe62e78726fd
0f85d163e6