MB-100: fix alerts test

Change-Id: I5727b09d069ab272d8e975a57f93ec19b52f983a
Reviewed-on: http://review.couchbase.org/25721
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>