Enable OpenCL testing via iris+clover in CI
This has caught a bunch of tricky fs/nir bugs so, once mesa/mesa!7047 (merged) lands, it's probably worth getting some coverage in CI. The OpenCL CTS in full is far too much; it takes days to run. There are also some piglit tests. We'll have to experiment a bit to see what gives us the best coverage for the lowest CI run-time cost.