Add unit tests to wheel building

[freddyaboulton]

The issue with the unit tests originally failing is that cibuildwheel will run the unit tests from the {package} directory, which in this case is prophet/python. This means that pkg_resources.resource_filename will look first in the local prophet/python/prophet/stan_model/ directory since the current directory is first in sys.path. You can sanity-check this logic by pip installing one of the wheels, cd-ing into prophet/python and running the unit tests. That should fail provided you don't have any compiled models in prophet/stan_model.

The solution I found was to run the unit tests from the package directory with pytest --pyargs prophet. I aslo added some cmdtanpy-backend unit tests to sanity check that.

You can see the unit tests passing on my for here