Tests are failing because we expect to see three different pids, but
only get two.  Is that because the work is being completed too quickly?

Coverage.py predates sets as a built-in data structure, so the file data
collection has long been dicts with None as the values.  Sets are
available to us now (since Python 2.4 in 2004, which coverage.py dropped
support for in 2014!), we use sets.

The comprehension would only completely finish (->exit) if the assert
was going to fail.  So we expect it to not finish.

The old pycache_prefix logic was because we would run tests in Docker
containers, and they went faster if we set PYTHONCACHEPREFIX.  But we
don't do that anymore, so we can use simpler code.