ci/piglit: Add some common piglit skips for Mesa CI's testing of glx.

ci/piglit: Add some common piglit skips for Mesa CI's testing of glx.

Since our X servers don't have a compositor, and we run tests in parallel,
various swap and frontbuffer tests won't ever be stable.  Rather than
having every driver have to track those flakes, make a general X11 skips
list as a known issue of our CI rather than pointing fingers at drivers.

Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Acked-by: Karol Herbst <kherbst@redhat.com>
Acked-by: Martin Roukala <martin.roukala@mupuf.org>
Acked-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20798>