BackupableDBTest thread-safe

Summary: We need to lock accesses to some TestEnv variables. Otherwise we get failures like http://ci-builds.fb.com/job/rocksdb_asan_check/657/console

Test Plan: make check

Reviewers: dhruba, haobo, sdong

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18489