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>
17 jobs for !1496 with ci-swrast in 19 minutes and 18 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Containers Build
passed #501931
debian

00:00:22

 
  Build+Test
passed #501940
meson-arm64

00:06:11

passed #501939
meson-armhf

00:02:19

passed #501933
meson-clang

00:09:21

passed #501937
meson-clover

00:09:39

passed #501941
meson-i386

00:03:15

passed #501936
meson-main

00:10:27

passed #501932
meson-swr-glvnd

00:07:58

passed #501938
meson-vulkan

00:03:38

passed #501943
scons-llvm

00:07:20

passed #501942
scons-nollvm

00:02:51

passed #501934
scons-swr

00:06:30

passed #501935
scons-win64

00:07:12

 
  Test
passed #501944
test-llvmpipe-gles2 1/4

00:04:55

passed #501945
test-llvmpipe-gles2 2/4

00:03:55

passed #501946
test-llvmpipe-gles2 3/4

00:04:50

passed #501947
test-llvmpipe-gles2 4/4

00:04:11