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Wow, I really botched that function. And no one seems to really use it to notice the errors (or at least the branches in the function where the errors exist are never reached). =)
So the tests should go in test_importlib or test_import (I'm leaning towards the latter since this function is for the C API). As for the files, I wouldn't create any and instead mock out importlib._bootstrap._path_isfile() to return the value you want.
And one last thing, Madison, is to do the patch against Python 3.3 if you can. This obviously needs to be backported so it's best to start there and then work forward.