Summary:
When run */RunMany/* tests individually, e.g. ChrootEnvWithDirectIO/EnvPosixTestWithParam.RunMany/0, they hang. It's because they insert to background thread pool without initializing them. Fix it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6931
Test Plan: Run ChrootEnvWithDirectIO/EnvPosixTestWithParam.RunMany/0 by itself and see it passes.
Reviewed By: riversand963
Differential Revision: D21875603
fbshipit-source-id: 7f848174c1a660254a2b1f7e11cca5370793ba30