Add tests for sheet2linkml

[gaurav]

The easiest way to test this would be to see if it can convert the CRDC-H model in Google Sheets into valid LinkML, but this is a bad idea since the CRDC-H model in Google Sheets will probably be changing soon. Instead, we should probably come up with a dummy spreadsheet whose sole purpose is to test this generator.

[gaurav]

Might be a good idea to do this for the data in https://harmonization.datacommons.cancer.gov/additional-resources

[gaurav]

• Once we do this, we should use GitHub Actions' "matrix" feature to test our code against multiple versions of Python.