ci/vc4: Add piglit run in VC4
These adds a couple of (manual) piglit jobs to execute in VC4. It excludes all the tests that causes GPU issues in the device, or that it takes too much time (>1 minute) to execute.
The number of jobs in parallel has been reduced also, as otherwise memory issues can appear while executing several tests in parallel, which could cause false positives.
This supersedes !8671 (closed)
/cc @anholt