Test Celery Beat regression (2.4.0 through 2.5.0 inclusive)

[szokeasaurusrex]

We had a regression in our Celery Beat integration that prevented Cron checkins from being sent at the end of the cron job from version 2.4.0 through (including) 2.5.0.

We isolated the cause to c80cad1, which we reverted in #3144. We released the fix in 2.5.1.

We should investigate the exact cause of this regression and add tests to ensure we cannot introduce a similar regression again.

[antonpirker]

The tests in the linked PR have been merged. This is done!