A change to assert_warns() was made on the release-4.0.x branch, which

was then backported to release-3.0.x. This was designed to improve

warning support, allowing testing against multiple warnings. However, it

also required that the checks be more specific, comparing warning

classes directly instead of checking subclasses, and comparing messages

directly instead of making them optional. This regressed a few tests on

release-3.0.x, which did not exist on release-4.0.x.
This change updates those tests to follow the new, stricter testing

requirements.

Unit tests pass.