r300,ci: add piglit testing

Runs the piglit gpu tests on RV530, runtime is around 25 minutes. Contrary to dEQP which is quite stable with the disabled HiZ, piglit flakes a lot more, here the flakes seems mostly related to tex-miplevel-selection.

I did stress test the job and it seems stable but I'll run it few more times before merging and keep an eye on it to see if we hit more flakes...