spec: make MessageableUserSpec more tolerant of non-clean db

Change-Id: I59a0588697c398677f2efa57a5c1d9b1117059e5
Reviewed-on: https://gerrit.instructure.com/29861
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>