Fix broken test in 31b02d.

Summary:
CorruptionTest for backupable_db_test did not call
GarbageCollect() after deleting a corrupt backup,
which sometimes lead to test failures as the newly created backup
would reuse the same backup ID and files and fail the consistency
check.

Moved around some of the test logic to ensure that GarbageCollect()
is called at the right time.

Test Plan:
Run backupable_db_test eight times and make sure
it passes repeatedly. Also run make check to make sure other
tests don't fail.

Reviewers: igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28863