docs: add test readability guidance

[kernelpanic888]

Summary

Add a short section on best practices for writing tests

Checks

• git diff --check
• .venv/bin/sphinx-build -W --keep-going -b html . _build/html -D language=en

Closes #509

[InessaPawson]

Thanks for your PR, @kernelpanic888! This aligns well with what @lwasser and @eliotwrobson discussed in #509. The section reads well and matches the voice of the rest of the page.

I have 2 suggestions for your submission:

1. Point the references to resources that are more specific to the subject of this PR. Currently, they link to the pytest landing page and the unittest module documentation, neither of which specifically addresses writing readable tests. (And the pytest homepage is already linked twice in this guide.)
2. Add a short good example right under the bullets.
   @lwasser suggested drawing on our packaging template output , which is what we teach in PyOS workshops. tests/run-tests.md already references pyos-package-template, so there is a precedent for pulling from it. An example would also ground the bullets in something package-specific, which addresses @eliotwrobson's concern about this part of the guide being generic.

[InessaPawson]

@lwasser @eliotwrobson I feel like this section would fit better below ### How do you know what type of tests to write? First, we discuss what tests to write, then move to how to write them well. Curious to hear your thoughts! 🙂

[InessaPawson]

@allcontributors please add @kernelpanic888 for doc

[allcontributors]

@InessaPawson

I've put up a pull request to add @kernelpanic888! 🎉