CBQE-2026: add UI multiply graceful failover tests

Change-Id: Ief30314fd92dadbd4bd5a30baf230dd80ab5a2e9
Reviewed-on: http://review.couchbase.org/35098
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>