feature: study how to use Spoon to implement the Arrange-Act-Assert pattern

The Arrange-Act-Assert pattern is meant to maximize readability of test code: https://java-design-patterns.com/patterns/arrange-act-assert/

This is more or less the same as the Given/When/Then pattern, see https://martinfowler.com/bliki/GivenWhenThen.html

We could use Spoon to rewrite Junit tests according to this pattern.

Related work:

* @jamescooke's https://github.com/jamescooke/flake8-aaa is Flake8 plugin that checks Python tests follow the Arrange-Act-Assert pattern