Summary:
A "LATEST_BACKUP" file was left in the backup directory by
"BackupEngineTest.NoDeleteWithReadOnly" test, affecting future test
runs. In particular, it caused "BackupEngineTest.IOStats" to fail since
it relies on backup directory containing only data written by its
`BackupEngine`.

The fix is to promote "LATEST_BACKUP" to an explicitly managed file so
it is deleted in `BackupEngineTest` constructor if it exists.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8887

Test Plan:
below command used to fail. Now it passes:

```
$ TEST_TMPDIR=/dev/shm ./backupable_db_test --gtest_filter='BackupEngineTest.NoDeleteWithReadOnly:BackupEngineTest.IOStats'
```

Reviewed By: pdillinger

Differential Revision: D30812336

Pulled By: ajkr

fbshipit-source-id: 32dfbe1368ebdab872e610764bfea5daf9a2af09