dufs,restic-rest-server: fix certificate renewal tests

The configuration of caucase used here is to have certificates valid for
3*31 days and renew them 31 days before the expiration date.
This test was running the updater 2 months later, which by chance was
62 days at the time of the test was written, because this test was
written in July (both July and August have 31 days), but this does not
work in other months.
Adjust the test to run the updater using a fixed number of days in the
future.