Fix some slow tests (and some other cosmetics)

anv_tests puts too much load on the system by inducing a ton of forks. When it's run in parallel with other tests, that's even worse. So make it a little bit less aggressive, and make sure it doesn't get run in parallel with other tests. The same for util_runner. And then a couple of small cosmetic tests from !30978 (merged) which have already been reviewed.