`group` and `setup_cron` integration tests currently install `libuser` and `faketime`
packages respectively, from the non-default edge/testing repositories. It is no
longer necessary to do so for the former so this patch drops the repo option. As
for the latter, this patch includes a note explaining when the same could be done
for it too.
This patch also adds a note on installing `faketime` on Alpine in CI.
(cherry picked from commit 619a1fa7e2)
3cffcfda9b