Summary:
Fsyncing files is not providing more test coverage in many tests. Provide an option in SpecialEnv to turn it off to speed it up and enable this option in some tests with relatively long run time.
Most of those tests can be divided as parameterized gtest too. This two speed up approaches are orthogonal and we can do both if needed.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7036

Test Plan: Run all tests and make sure they pass.

Reviewed By: ltamasi

Differential Revision: D22268084

fbshipit-source-id: 6d4a838a1b7328c13931a2a5d93de57aa02afaab