Add initial FastAPI unit tests

## Summary

We need to include unit tests for the backend. Eventually we would like to have all endpoints covered and most logic. For now, we simply need to create the initial tests and include some examples of how to structure tests and how to run via pytest.

# Acceptance Criteria

1. Demo tests exist for endpoints, "business logic", and models and can be run using pytest
2. Pytest tests are executed as part of the build pipeline