MDL-87034 hub: Reset hub registration caches during tests

public/lib/classes/hub/registration.php

@@ -961,4 +961,11 @@ class registration {
         $size = $size / (1024 * 1024);
         return round($size, 3);
     }
+
+    /**
+     * Resets the static caches for unit tests.
+     */
+    public static function reset_caches(): void {
+        self::$registration = null;
+    }
 }

public/lib/phpunit/classes/util.php

@@ -245,6 +245,9 @@ class phpunit_util extends testing_util {
             core_calendar\local\event\container::reset_caches();
         }
 
+        // Reset hub registration caches.
+        \core\hub\registration::reset_caches();
+
         // TODO MDL-25290: add more resets here and probably refactor them to new core function.
 
         // Reset course and module caches.