Summary:
`DBTest.FileCreationRandomFailure` frequently times out during our
continuous test runs. (It's a case of "stress test posing as unit test.")
The patch reduces the number of iterations to avoid this. Note that
the lower numbers are still sufficient to trigger both flushes and
compactions, so test coverage is still the same.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7481

Test Plan: `make check`

Reviewed By: riversand963

Differential Revision: D24034712

Pulled By: ltamasi

fbshipit-source-id: 8731a9446e5a121a1041b00f0df473b9f714935a