Check logs when locking and unlocking GNOME shell

Current GUI tests, specifically those in local-user-lock-on-removal.py, do not check any logs, when locking and unlocking GNOME shell. According to [superuser forum](https://superuser.com/questions/662974/logging-lock-screen-events), GNOME shell does not generate any logs, but it does send DBus messages. Further investigation is needed.