Summary:
`DBTest::SnapshotFiles` runs the tests in a `while` loop.
Currently, the snapshot directory is not cleaned up in each loop, so previous snapshot files may remain in the next loop's snapshot.
When I'm working on https://github.com/facebook/rocksdb/pull/7129, when checking the tracked WALs in MANIFEST, I find that this test always fails because it reads some unknown WAL. It turns out that the unknown WAL is left from previous loops.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7156

Test Plan: make db_test && ./db_test --gtest_filters=*SnapshotFiles

Reviewed By: siying

Differential Revision: D22668360

Pulled By: cheng-chang

fbshipit-source-id: 69d4aa3506038ba30e218e8ae966357935a99c6c