autotests: Skip testScreencasting in CI
pipewire 1.2 has a known regression where the client's process callback will not be triggered if the producer has sent only one buffer.
The issue has already been fixed upstream https://gitlab.freedesktop.org/pipewire/pipewire/-/commit/525360d70ab1698afaaaf20f7e58002b8756353f
Until our CI picks up that change, temporarily disable the test to unblock merging patches.