Go back to single core for test suite on CI (#2305)

The random asyncio bug is just too frequent and annoying to be
worth the speed improvements. Our test suite is already quite fast.
Random test failures hurt for 3 reasons, 1) they are discouraging for
new contributors who won't understand it's out of their control, 2)
it's annoying and time consuming to rerun the workflow, and 3) it
makes single job failures feel less important (even they should be
treated as important!).