Document how to run unit tests with an API key

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I didn't dig deep in the tests but I guess it's defaulting to OpenAI API.
Just running python -m unittest discover -s tests, bumps in WARNING:openevolve.llm.openai:Error on attempt 1/4: Error code: 401 - {'error': {'message': 'Incorrect API key provided: test. You can find your API key at https://platform.openai.com/account/api-keys.', 'type': 'invalid_request_error', 'param': None, 'code': 'invalid_api_key'}}.
Maybe it's good to add the fact that we need the environment variable to CONTRIBUTING.md or even better to README.md

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Even better, would be to load this from .env and also be able to set provider and models (e.g. im comfy with openrouter :) )