MDL-48605 unit_test: Fixed file_temp_cleanup_task

touch should only work on temp and below
dir's. With path we get temp/.. and temp/.
So we were modfying moodledata dir as well
and it was failing because of permission issues.

lib/classes/task/file_temp_cleanup_task.php

@@ -88,8 +88,10 @@ class file_temp_cleanup_task extends scheduled_task {
                     }
                 }
             } else {
-                // Return the time modified to the original date.
-                touch($node, $modifieddateobject[$node]);
+                // Return the time modified to the original date only for real files.
+                if ($iter->isDir() && !$iter->isDot()) {
+                    touch($node, $modifieddateobject[$node]);
+                }
             }
         }
     }