Summary:
FaultInjectionTestFS::NewDirectory currently asserts that the directory
creation on the target filesystem succeeds. This is actually not
guaranteed since there might be a legitimate I/O error when creating the
directory. The patch removes this assertion.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7220

Test Plan: `make check`

Reviewed By: zhichao-cao

Differential Revision: D22957990

Pulled By: ltamasi

fbshipit-source-id: b2e221320d8ce7235cb4897ef5936072412a25b6