FWIW unittest had a similar issue and it's been solved adding an 'exit' argument to unittest.main() [0].

I think using an attribute here might be fine.
The patch contains some trailing whitespace that should be removed, also it might be enough to name the attribute "exit_on_error".
It should also include tests to check that the attribute is set with the correct default value and that it doesn't raise SystemExit when the attribute is False.

[0]: http://docs.python.org/library/unittest.html#unittest.main