MDL-77837 phpunit: Ensure that the cron user setter is used

When running an adhoc task in a unit test we should use the cron variant
of the set user method to mimic the behaviour of a real cron run.

lib/phpunit/classes/advanced_testcase.php

@@ -733,7 +733,7 @@ abstract class advanced_testcase extends base_testcase {
             }
 
             cron_prepare_core_renderer();
-            $this->setUser($user);
+            cron_setup_user($user);
 
             $task->execute();
             \core\task\manager::adhoc_task_complete($task);