ci: new CTS runner, full softpipe gles3/31
This switches us over to the new CTS runner so that we can use threads within a job to keep the cores busy (and get softpipe/llvmpipe done much faster), and will help us tune freedreno throughput and avoid spurious fails as we move to enabling vulkan testing (which will inevitably involve some scribbling on other process's memory).
Edited by Emma Anholt