gitlab-ci: Run the GLES2 CTS on llvmpipe.

gitlab-ci: Run the GLES2 CTS on llvmpipe.

This is the start of doing CTS tests on merges to Mesa master.  We use
the surfaceless platform so that we don't need to bother bringing up
weston or X11.  The surface size is kept low to reduce runtime, but
this comes at the cost of many rendering tests skipping due to
too-small render targets (as we see the impact of Mesa on the shared
runner pool, we can reevaluate this and what set of CTS tests we want
to run).

We split the job up across 4 runners (each at 4 llvmpipe threads), so
that the job can load-balance across our shared runners and finish
sooner (since dEQP is very single-thread-performance bound).

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>