Refactor tests that depend on API calls

[rvhonorato]

The results of some of the current tests can be affected by unwanted artifacts since they are also acting as sanity tests for the API endpoints, see https://github.com/haddocking/arctic3d/actions/runs/8892529851/job/24416781299?pr=407

For example the test below will fail if the response from the API changes:

https://github.com/haddocking/arctic3d/blob/7082f8586693c93b09a4d7e2f1b1b7b33c0e33de/tests/test_interface.py#L124-L142

This can be fixed by adding a sanity test screening to the endpoints;

• https://www.ebi.ac.uk/proteins/api/proteins
• https://www.ebi.ac.uk/pdbe/graph-api/uniprot/interface_residues
• https://www.ebi.ac.uk/pdbe/graph-api/uniprot/ligand_sites
• https://www.ebi.ac.uk/pdbe/graph-api/mappings/best_structures
• https://www.ebi.ac.uk/pdbe/entry-files/download

And mocking their response in the unit-tests