Add tests to check validity of performance measures with multiple resources?

[amyheather]

By design, this is a single resource model. But when I've been writing the code, I've been trying to do it with the idea in mind that it could be extended for models with multiple resources.

Idea: Adding test with a multi-resource model checking validity of performance measures.

Linked issue: pythonhealthdatascience/pydesrap_mms#67