docs: add test readability guidance

[kernelpanic888]

Summary

• Add a short section on writing tests that are easy for reviewers and contributors to read.
• Place it after the existing test_add_numbers example in tests/write-tests.md.
• Include maintained testing references for pytest and Python's unittest docs.

Checks

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

Closes #509