The desktop_notifications tests called the real init/alert/show, which
reach the libnotify daemon — every test run popped real GNOME
notifications titled "MockDevice" and "unknown".
Add an autouse conftest fixture that swaps Notify for a mock in both
desktop_notifications modules (solaar.ui and logitech_receiver). The
tests still exercise the real init/alert/show code paths, but
Notification.show() never reaches the daemon. The fixture returns the
mock; the notification tests now assert show() was actually dispatched
against it, so they verify the path instead of just "didn't crash".
4c709ecb73