Our TestCase.assert_warns method only worked when a single warning was

being emitted by code. There wasn't much reason for this, besides of

ease of implementation, and it made unit tests more fragile.
This change updates the function to check the provided warning

information against all emitted warnings, looking for a match. If none

are found, this will fail with an explicit error message.

Unit tests pass.
Made use of this in an upcoming unit test, which has to deal with multiple

warnings. It passed.