Tests: Combine duplicate `update_posts_count()` tests.

This combines the newer test for `update_posts_count()` located in its own file with the pre-existing one from `tests/multisite/site.php`, which was essentially testing the same thing in a similar way.

Includes:
* Renaming the test class per the [https://make.wordpress.org/core/handbook/testing/automated-testing/writing-phpunit-tests/#naming-and-organization naming conventions].
* Adjusting comments per the documentation standards.
* Updating `@covers` tags for accuracy.
* Removing unnecessary blog switching.
* Using `assertSame()` to check the value type.

Follow-up to [28835], [29667], [52207].

See #57023, #56793.
Built from https://develop.svn.wordpress.org/trunk@54760


git-svn-id: http://core.svn.wordpress.org/trunk@54312 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.2-alpha-54759';
+$wp_version = '6.2-alpha-54760';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.