How to test 'init' hook?

As we run code in `wp-settings.php` we execute a lot of actions.

```php
do_action( 'plugins_loaded' );
do_action( 'sanitize_comment_cookies' );
do_action( 'setup_theme' );
do_action( 'after_setup_theme' );
do_action( 'init' );
do_action( 'wp_loaded' );
```

How to test code hooked to e.g. `after_setup_theme`?

Start by reading wp-settings.php and the listed lifecycle hooks, especially after_setup_theme and init, then inspect the existing PHPUnit test setup. The issue does not define a desired testing API or a concrete success case, so clarify the expected way to execute and assert hooked code before assessing what done means.

Written by the indexing model from the issue text.