NCBC-388: Improve NUnit Tests : DeleteBucket() should be called before checking that the bucket is deleted and is no longer listed.

Change-Id: I7278b3e9bb653632a2458253370f81b9df3c690f
Reviewed-on: http://review.couchbase.org/34736
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>