the main code is appended in main.py

if a test uses with self.subTest(), subTest() fails right in its first statement "if not self._outcome ..." because _outcome is None.

In main.py, the commented runner.run(suite) would work correctly.

If this is not meant to work, please mention it in the documentation and  improve unittest's reaction.

what I would find more logical from the users's point of view is something like

runner.debug(suite)