Implement unit tests for gwas.py

[ttfiliz]

Good to implement unit tests for gwas.py, following https://github.com/comorment/containers/blob/main/usecases/gwas_demo.md (but just the part that invokes gwas.py, not the part that executes the actual gwas)

[github-actions]

Hello @ttfiliz , thank you for submitting an issue!