Tests: Use a consistent way of setting the Administrator role in Ajax…

… tests.

This removes some duplicate code in favor of calling the `WP_Ajax_UnitTestCase::_setRole()` method created specifically for this purpose and used in other tests.

Follow-up to [500/tests], [37288].

Props martin.krcho.
See #56203.
Built from https://develop.svn.wordpress.org/trunk@53701


git-svn-id: http://core.svn.wordpress.org/trunk@53260 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53700';
+$wp_version = '6.1-alpha-53701';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.