Resolver: introduce same test coverage as loader

[clebergnu]

This introduces a port of selftests/unit/test_loader.py to cover the resolver. The idea is to have simultaneous coverage until the removal of the loader code.

[clebergnu]

@richtja I'm getting this back to draft. It seems like there's a very weird issue with the usage of unittest.mock that leaks the behavior into other tests when using Python's standard unittest runner (used in the coverage job).