MDL-77237 core: Suppress debugging during capabilities cleanup

To prevent the debugging messages from appearing during the upgrade,
I set a flag to hide them.

For the test file, I adjusted the parameters in
the unassign_capability() function, which seemed to be using
the parameter intended for assign_capability().

lib/classes/param.php

@@ -1189,7 +1189,7 @@ enum param: string {
      * @return string
      */
     protected function clean_param_value_capability(mixed $param): string {
-        if (get_capability_info($param)) {
+        if (!empty($param) && get_capability_info($param)) {
             return $param;
         } else {
             return '';