Tests: Move the `wp_list_pages()` test for CSS classes to a more appropriate place.

Back when this test was introduced, `wp_list_pages()` did not have its own test class.

It does now, so the test can be moved there, instead of being hidden among `get_pages()` tests.

Includes:
* Updating the test name for clarity.
* Adding an unique message for each assertion.

Follow-up to [27755], [28400].

See #57841.
Built from https://develop.svn.wordpress.org/trunk@55588


git-svn-id: http://core.svn.wordpress.org/trunk@55100 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.3-alpha-55584';
+$wp_version = '6.3-alpha-55588';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.