Summary:
Several of backupable_db_test fails if running standalone, because of directory missing. Fix it by:
(1) garbage collector skips shared directory if it doesn't exit
(2) BackupableDBTest.Issue921Test to create the parent directory of the backup directory fist.
Test Plan: Run the tests individually and make sure they pass
Subscribers: leveldb, andrewkr, dhruba
Differential Revision: https://reviews.facebook.net/D56829
cea8ed9702