Tests: Refactor `Tests_Image_Functions::test_load_directory()` to split the tests and use a data provider.

This one test was testing three different situations. When one assertion fails, the rest of the test would not be executed, so this leads to hiding one error behind another.

Splitting the test into three distinct test methods still allows for testing each situation, but tests each one in isolation and won't hide errors.

The third part of the test, dealing with image editor engine classes, will also now use a data provider.

Using a data provider has a number of advantages:
1. If the first test case fails, it won't prevent the other test cases from being tested.
2. The output from PHPUnit will be more descriptive in case of failure when using a data provider.
3. Using named test cases in the data provider will also make the `--testdox` output much more descriptive and informative.

The actual cases being tested, or the test itself have not been changed.

Includes:
* Adding `@covers` annotations.
* Adding a failure message to each assertion when multiple assertions are used in the test.
* Reusing an existing data provider with the available image editor engine classes.

Follow-up to [1061/tests], [53495], [53497], [53521], [53523], [53524], [53525], [53526], [53529], [53530], [53531].

Props jrf.
See #55652.
Built from https://develop.svn.wordpress.org/trunk@53537


git-svn-id: http://core.svn.wordpress.org/trunk@53126 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53536';
+$wp_version = '6.1-alpha-53537';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.