refactor: remove redundant map_demand_to_buses

[danielolsen]

Purpose

• Remove redundant function which gets bus demand. See performance comparison in refactor: remove redundant function for getting bus demand PowerSimData#388.
• Refactor the congestion surplus tests from unittest to pytest to get to monkeypatch functionality (& general succinctness)
• Refactor test of congestion surplus calculation to monkeypatch around the InputData.get_data method which relies on profile CSVs. (see https://docs.pytest.org/en/stable/monkeypatch.html)

Time estimate

10 minutes, mostly for understanding the test changes.

[BainanXia]

The test refactor is clever. Thanks.

[danielolsen]

The test refactor is clever. Thanks.

I don't remember how I learned that monkeypatch exists, but @jon-hagg used it in PowerSimData so maybe that was it. I think there are many places in our code that could benefit from this, places where our main code is complex to make testing easy, and it would be better to make the main code simple and do something like this for the tests.