gitlab-ci: Enable the GLES2/3 CTS on softpipe.

gitlab-ci: Enable the GLES2/3 CTS on softpipe.

The GLES2 CTS takes about 8 minutes of total runtime (at parallel 4 is
~2 minutes in the test stage if runners are free), while GLES3 takes
about 25.  Since the GLES3 run is pretty expensive, just do a cheap
touch test of 1 out of every 10 tests in the test list on MRs, until
we can get the runtime down.

v2: Drop the full run for now until we can bring runtime down or bring
    up a dedicated mesa runner.

Reviewed-by: Eric Engestrom <eric@engestrom.ch> (v1)
Reviewed-By: Gert Wollny <gert.wollny@collabora.com> (v1)