Evaluate Ponytail for keyboard input in the automated test suite

Especially since migrating to Wayland we experience things like lost keypresses/modifier keys with the libvirt-based keyboard input (#19237) so let's see if Ponytail's keyboard input has better luck with its remote desktop-based approach.

Limitation: Ponytail is only available in the GDM and GNOME sessions, so we still need the libvirt-based keyboard input before that in the boot process.