Run tests in parallel

[matheper]

• Fix a small bug when calling the library Engine directly in tests (it was creating a path that includes the prefix local:)
• Add docstring to BackendEngine warning to not use the class directly and that repo_id should not include prefix (it works correctly when called form LibraryExpert classes)
• Running pytest using 4 processes (max n. processes for public repos). Doesn't seem to help much but doesn't make it worse either. I will keep the 4 proccesses config
• black / isort changes (I am creating a second PR setting up an action to run them automatically)