ci: Re-enable some swrast testing using fd.o's shared runners for now.

ci: Re-enable some swrast testing using fd.o's shared runners for now.

I'm not planning to stand mesa-swrast back up until we get Kata set up, so
turn the testing back on at a reduced fraction on so that
venus/llvmpipe/etc. dev can still get some coverage.

I haven't turned lavapipe back on, because it is now unstable in memory
model / atomics tests.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21880>