Need to add "preview_namelists" unit tests in order to test buildnml

Currently there are no unit tests for `buildnml` in the automated testing framework, nor are there any integration tests for the complete model configuration/code generation/namelist generation system.

To resolve this issue, it would be beneficial to create some tests where the model is setup all the way through the `preview_namelists` step, and then the resulting generated files are checked to make sure they match what is expected.

Also, given that all of this will theoretically go through only the python code, the tests should be easily runnable in a Github Actions workflow. This workflow can also be designed such that it only runs if the standard python unit tests workflow passes successfully.