CV failure to remove benchmarking directory upon completion

What's the issue?
We do some simple performance testing in our CV, it transiently fails to remove the test directory due to it not being empty (this is a race condition).

What's the fix?
Ensure that we close the vBucketBackupWriter before we try to remove the test directory; this ensures the metawriter goroutine doesn't attempt to persist any data whilst we're actively removing the test directory.