spec: fix finding last message for mysql

refs CNVS-7032

Change-Id: I6d41fc21e287e177538f96dd387e06cb3696140f
Reviewed-on: https://gerrit.instructure.com/28784
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>