MB-100: minor improvement in failover tests

Change-Id: I816a1b0c0e9273a305d3b4f4dc9650773af5ea0f
Reviewed-on: http://review.couchbase.org/24470
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>