selftest: disable virgl headless test

Crashes in mesa when using vgem kernel module (in the absence of real GPU)
have been observed in dunfell (they do not happen in master):

https://bugzilla.yoctoproject.org/show_bug.cgi?id=14527



Let's focus on making host-accelerated virtualized graphics work well in master
and upcoming releases (the issue is not seen there).

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>