refactor tests to be less implementation-dependent

[pldorn]

Description

A proposed refactor of the clean_all_years python unit tests. This version is less dependent upon the current implementation in clean_and_pare_down_data_all_years.py. The motivation is to make future refactors in the data processing code easier.

This approach largely focuses on a single fixture that wraps the results from the process method of clean_and_pare_down_data_all_years.py and then tests expected behavior against it.

Testing Instructions

• pytest output is clean: docker-compose run --rm electrify-chicago python -m pytest

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes

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