Summary: Fix BackupableDBTest.NoDoubleCopy and BackupableDBTest.DifferentEnvs by mocking the db files in db_env instead of backup_env_
Test Plan: make check -j64
Reviewers: sdong, andrewkr
Reviewed By: andrewkr
Subscribers: andrewkr, dhruba
Differential Revision: https://reviews.facebook.net/D57273