update merge proxy specs to be tolerant of non-empty db

Change-Id: If253e13b9a1ebfe91b72b25d357f798800397047
Reviewed-on: https://gerrit.instructure.com/28815
Reviewed-by: Jon Jensen <jon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>