cleaned up DelayedAlertSenderSpec

fixes: CNVS-12476

test plan:
- no QA needed, spec changes only

Change-Id: Ic7d4340acfcc27c8b24d461856d3eca61c7853d7
Reviewed-on: https://gerrit.instructure.com/33394
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>