refactor tests for Alert model

fixes CNVS-12353

test plan:
- no QA needed, only specs changed

Change-Id: Idb59de448446c07242f0150cf265d02efd8b7ff0
Reviewed-on: https://gerrit.instructure.com/33219
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>