Tests: Rename test class and improve tests for wp_get_global_stylesheet().

Changes include:
* Renames the test class to be compliant with test coding standards.
* Converts the test class to extend `WP_Theme_UnitTestCase` to reuse test fixtures.
* Implements data providers to encapsulate datasets and reduce code repetition.
* Adds a `@covers` annotation.
* Improves assertion messages to help with diagnosing failed tests.

Follow-up to [55567], [55148], [52682], [53916], [52675-52677].

Props costdev, hellofromTonya.
See #57841, #57958.
Built from https://develop.svn.wordpress.org/trunk@55572


git-svn-id: http://core.svn.wordpress.org/trunk@55084 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.3-alpha-55571';
+$wp_version = '6.3-alpha-55572';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.