CBQE-0: minor improvements in failover tests

Change-Id: Ib59fea03db9e01cdededdfac0a2c332534290a6c
Reviewed-on: http://review.couchbase.org/35065
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>