Run ratiotests by default

[ammarcsj]

This PR adds end-to-end tests that check the protein ratio distribution:

• directLFQ is called on two different mixed species datasets (1 Bruker, 1 Thermo)
• New RatioChecker class checks protein fold changes for each species (e.g. Yeast, Human)
• Tests run by default on every PR
• python version changed to >=3.9

These tests verify that protein fold changes behave as expected across different species in mixed datasets.