[tests] consider whether and how to test __main__.py and gptty.py

[signebedi]

[tests] consider whether and how to test main.py and gptty.py
Currently, we are testing a number of functions, but not testing main.py and gptty.py. These are mostly click interfaces and gptty-specific functions. Is it worth testing these? If so, how?

Originally posted by @signebedi in #40 (comment)

[signebedi]

If we want to test these features, we'll need to add a OpenAI API key as a github actions secret, see https://docs.github.com/en/rest/actions/secrets. This will allow us to test the functionality of the actual completions.