Use gdm3 instead of lightdm, and ensure udev is up-to-date.

Sometimes, udev is not upgraded in the testbed when testing new systemd. Ensure, new udev is always installed as sometimes changes to common code may lead to incompatibilities between udevd and for example networkd which are shipped in two different binary packages.

Replace lightdm based tests with gdm3. gdm3 is now a more representative test-case of a working default graphical display manager.

Install more per-arch qemu-system-ARCH packages.