Verify that files are included or not included in source distributions and wheels

[namurphy]

With #2598, we moved to an src layout with a separate tests directory. There is a chance that there will be some lingering problems with source distributions and wheels not including the correct files. Before we release 2024.5.0, I will need to look through what files are included in the sdists and wheels and make sure that everything is as it should be.

Ideally, we should include tests/ in sdists but not in wheels.