Only load \phpunit_util if not autoloadable

From Moodle 5.2 the `\phpunit_util` class has been renamed to
`\core\test\phpunit\phpunit_util`, supports the autoloader, and can
still be autoloaded from its old location.

Attempting to manually load the file leads to a failure.

Author: andrewnicols

verify_phpunit_xml/create_phpunit_xml.php

@@ -92,7 +92,9 @@
 // Now, let's invoke phpunit utils to generate the phpunit.xml file
 
 // We need to load a few things manually.
-require_once("{$dirroot}/lib/phpunit/classes/util.php");
+if (!class_exists(\phpunit_util::class)) {
+    require_once("{$dirroot}/lib/phpunit/classes/util.php");
+}
 
 if (!class_exists(\core\output\core_renderer::class)) {
     // From Moodle 4.5 we start to autoload the output components and including outputcomponents.php will throw an exception.