Tests that creating and starting a virtual screen cast monitor works,
and that at least one one buffer is processed.
Currently the content of the buffer isn't checked more than it can be
mmap():ed. Only MemFd buffers are tested for for now, as DMA buffers
would need a surfaceless EGL context to check properly.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1698>
