FAILED tests/patcher_test.py::test_fork_after_monkey_patch - UnicodeDecodeError: 'utf-8' codec can't decode byte 0xaa in position 261961: invalid start byte happens sometimes in py38-asyncio runs. The result of this error is that the output doesn't get logged, so it's unclear what the root cause is.

            ok = lines[-1].rstrip() == b'pass'
            if not ok or len(lines) > 1:
>               sys.stderr.write('Program {} output:\n---\n{}\n---\n'.format(path, output.decode()))
E               UnicodeDecodeError: 'utf-8' codec can't decode byte 0xaa in position 261961: invalid start byte

We should change the logging so it's more resilient to garbage, so we can diagnose the intermittent root issue.