ci/piglit: Skip glx_arb_sync_control@timing.* on all systems.

ci/piglit: Skip glx_arb_sync_control@timing.* on all systems.

The test involves timestamping to figure out how long a swap actually
takes, but if anything ends up rescheduling the process you can end up
spuriously failing.  I could easily reproduce flakiness by just running a
loop accessing the filesystem in parallel with a loop running the test.
So, it's certainly not usable on a CI system with other piglit tests
running in parallel, and we don't want to run it if it's going to just
produce flake noise.

Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11333>