Tests: Correct the @covers tag in a WP_REST_Posts_Controller test for unique post slugs.

This addresses a notice when generating the code coverage report:
{{{
"@covers WP_REST_Request::create_item" is invalid
}}}

The `WP_REST_Request` class does not have a `create_item()` method, `WP_REST_Posts_Controller` is the class being tested here.

Includes fixing a typo in the test method name.

Follow-up to [53813].

See #52422, #55652.
Built from https://develop.svn.wordpress.org/trunk@54055


git-svn-id: http://core.svn.wordpress.org/trunk@53614 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-54054';
+$wp_version = '6.1-alpha-54055';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.