tests: start dbus-display-test

Cover basic display interface usage. More cases to be added to cover
disconnections, multiple connections, corner cases. At this point, they
would be better written in Rust or Python though.

The proxy also covers reading the properties, since they are
automatically loaded at creation.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>