Do some tests whether dnf needs-restart correctly handles kernel updates and how it reports them in the output.

It currently has a ‐‐services mode which is fine, but it does not filter out these processes from services from the --useronly mode. At least this needs to be reported, and is a blocker for fully migrating to it: https://issues.redhat.com/browse/RHEL-56137

Updating the kernel does not appear in dnf needs-restarting default output, but it does appear in --reboothint. It's hard to machine read, though, filed as https://issues.redhat.com/browse/RHEL-56139:

# dnf needs-restarting --reboothint
Core libraries or services have been updated since boot-up:
  * kernel
  * kernel-core

Reboot is required to fully utilize these updates.
More information: https://access.redhat.com/solutions/27943

I also checked TestUpdates.testKpatch and nothing appears in --reboothint. I can't say whether that is because of cleverness or ignorance of kpatches in dnf. Unfortunately I don't know how to test this with real kpatch updates, as they are so inaccessible. However, the worst that can happen is that Cockpit offers a reboot; the admin can still look at the update list and instead decide to just continue. So I think that's ok for now.