Add tests with pytest

[matthewfeickert]

A few test cases for sanity sound good. I only tested by hand. Do you think we should have a fully functional LaTeX env and some pytests with file comparisons?

Originally posted by @duetosymmetry in https://github.com/duetosymmetry/ads2inspire/issues/11#issuecomment-738347269

[duetosymmetry]

Thanks for opening the issue, @matthewfeickert!

• [ ] Decide on a set of test cases, at least one test for each feature
• [ ] Create minimal .tex and .bib file(s) which will cover all test cases
• [ ] test_* functions (with file comparisons) that pytest will run
• [ ] Add a github action (likely using @xu-cheng's https://github.com/xu-cheng/texlive-action and https://github.com/xu-cheng/latex-docker, as originally mentioned by @matthewfeickert in https://github.com/duetosymmetry/ads2inspire/pull/11#issuecomment-738376880)