On a second look, there are a couple of things missing:
- the default value didn't actually get changed!
- we should add an explicit test that the no argument behaviour doesn't raise a warning by default

Since we don't really have an existing test suite for the support code, I'd suggest coopting test_sundry.py for the purpose.

I agree that an exception should be raised if the user passes in an expected warning but no warning at all is encountered.