CBQE-1258: failover tests hang on WorkerThread

 if buckets were deleted from other jobs/tests

Change-Id: Idcc29b65d850318a31e10e1f102553e78e8291dc
Reviewed-on: http://review.couchbase.org/25965
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>