Summary:
When DestroyDB() finds a wal file in the DB directory, it assumes it is actually in WAL directory. This can lead to confusion, since it reports IO error when it tries to delete wal file from DB directory. For example: https://ci-builds.fb.com/job/rocksdb_clang_build/296/console

This change will fix our unit tests.

Test Plan: unit tests work

Reviewers: yhchiang, rven, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D32907