panfrost/ci: Smoke test spilling

Spilling is tricky and doesn't get much testing in CI. Run
a subset of dEQP-GLES2.functional.shaders.* with spilling forced to get spilling
tested in CI.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <!16314>