spec: fix delayed jobs specs on mysql dependent on subsecond precision

refs CNVS-7032

Change-Id: I6ac3cba00d45bccec6c0c8067ff43a2b0c21161d
Reviewed-on: https://gerrit.instructure.com/28808
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>