On the Delicious Brains blog there's a post sharing some of their knowledge about building tests for WP-CLI packages, a set of command line tools for administering a WordPress installation. Their testing makes use of the Behat testing tool (already in use on WP-CLI's own tests).

My last article was part of a short series on automating local WordPress site setup. In that series, we created a WP-CLI package that helps with installing and uninstalling WordPress development environments, and we even got it submitted to the WP-CLI Package Index.

[...] In this post we’re going to take a bit of a break from automating WordPress installs and start writing some functional tests to make sure that everything works as expected. While I’ll be writing the tests for the wp installer command, the same concepts should apply for any WP-CLI package.

They start by clarifying the difference between functional and unit tests and how to get your environment all set up and ready to use for testing. They help you get the wp_scaffold_package installed and how to confirm that everything is working as expected. From there it's all about the tests: ensuring that a package is active, creating a custom step to use in testing and an example of what the output should look like.