Re-introduce PackageKit cancellation test

This was removed in https://github.com/cockpit-project/cockpit/pull/23719 as it leads to flaky/often breaking packagekit tests.

The cancellation is hard to test reliably, https://github.com/cockpit-project/cockpit/pull/23719#discussion_r3988176703 but we should try to revive this by serving the packages either from a slow mirror or slow down the disk.