autopkgtest: make resolved optional

systemd-resolved is no longer in unstable, so some of the autopkgtest suites are failing. Adjust the dependency as bullseye is no longer relevant (EOL), and mark the affected tests as skippable so that they are gracefully skipped over.

I apologize for the disruption. Unfortunately when features are dropped, some disruption is inevitable. As far as I can tell netplan.io works perfectly fine without systemd tools and services, so I do not know why these tests require resolved. Unfortunately I am not familiar with the implementation, so the quick and naive fix is to mark the tests as skippable (the dependency is dropped where not needed). This should make netplan.io able to migrate again. Ubuntu should not be affected at all.