Summary:
Introduced `valgrind_check_some`, which is analogous to the `check_some` target for non-valgrind tests. It simplifies the process for running a single valgrind test or subset of valgrind tests when trying to repro a failure.

I also added a `ROCKSDBTESTS_ONLY` parameter, which simplifies selecting a single test to run. Previously the user would have to use `ROCKSDBTESTS_START` and `ROCKSDBTESTS_END`, but it was difficult to determine the end variable since it is an exclusive endpoint and must match an actual test name.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7379

Reviewed By: pdillinger

Differential Revision: D23673608

Pulled By: ajkr

fbshipit-source-id: 87ed81f1a671d46c2dff6a701f85f1891c725b3f