(#2251) Make the crontab integration spec match the behavior again

The previous final cron test was Very Bad. It relied on an edge case
that sidestepped #2251.

Assume user1 and user2 and a cronjob X that moves from user1 to user2.
Normally, this will trigger #2251 and fill user2's crontab with duplicates
until at some point, another cronjob of user1's needs changing and the
provider behaves as tested.

In many real life scenarios, it can be months or weeks before this happens.

Testing for the behaviour agreed upon in the redmine ticket makes more
sense, because we can now make sure that the edge case will not hide
unwanted behavior that emerges in the average case.