Summary:
The patch fixes the build for `db_bench_tool_test` and makes the tests pass.
Namely, it fixes the following issues:

* https://github.com/facebook/rocksdb/issues/7703 removed the member variable `fs_` but the test case `OptionsFileMultiLevelUniversal`
was not updated.
* https://github.com/facebook/rocksdb/issues/7344 fixed the `OptionsFile` test case for the case when Snappy is *not* available but at the
same time broke it for the case when it *is* available. (The test used a default-constructed
`ColumnFamilyOptions` object, and the default value of the `compression` option is either
Snappy or no compression depending on whether Snappy is supported.)
* The test used `google::ParseCommandLineFlags` instead of
`GFLAGS_NAMESPACE::ParseCommandLineFlags`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7935

Test Plan: Ran the test both with and without Snappy support.

Reviewed By: zhichao-cao

Differential Revision: D26269765

Pulled By: ltamasi

fbshipit-source-id: b7303d8a981ab299d22ab540e0cbd12d149ed9bb