fix spec to be more lenient for no sub-second timestamps in mysql

refs CNVS-7032

Change-Id: I00b6d7ea6d6a2d2c35fb9bc2870e2872b3482207
Reviewed-on: https://gerrit.instructure.com/28778
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>