Ah, I see. The setup/teardown stuff runs for each test.

So this is what I did:
* Added a method to add a "bad" source file to the source directory. It gets cleaned up with the existing teardown method.
* Used test_importlib to temporarily mutate sys.path as you recommended.

I think this is much closer to what we want. Let me know what you think.

By the way, are there any docs on test_importlib? I couldn't find any.