spec: improve AR::Base specs tolerance of non-clean db

Change-Id: I0e110941c8733a8ca90d03570b8bfd0e1749f7af
Reviewed-on: https://gerrit.instructure.com/29054
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>