Previously, when assertWarnings() found fewer or more warnings than

expected, the only result we'd get was an assertion error showing the

mismatched lengths.
Now, any mismatches between expected and actual warnings will result in

a full list of the mismatches, helping with diagnosing which warnings

were missing or added.

Unit tests pass.
Tested this with an assertion error I hit. Verified I got a more useful

warnings display.