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