ci: Test the KHR-GL* CTS cases with softpipe.

I've been hacking on softpipe in the process of trying to delete a bunch
of core Mesa code, and want to make sure I don't regress desktop GL
either.  The run takes under a minute and a half.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6466>
3 files changed