Perhaps an add-on in the form of a momentary push-button? Soldered any place you want to trigger an alert? Example, sensor leads. Press the switch to mimic failure of the sensor.
As you know it's a good idea to have backup components, such as two duplicate sensors. If you can arrange to test one at a time, then it causes a warning, but not a false alarm.
Or a reed switch. It closes when you bring a magnet near. Such a switch can be hidden behind a panel (of non-ferrous material). No need to drill holes.
Or fiber optics carrying light to photosensors. These are installed in places where you want to switch something on or off as a means of testing.
Behavior might be affected by a magnet held close, or moved back-and-forth.
The idea of automating tests is appealing however it sounds labor-intensive, requiring you to install cables or fibers between equipment across the facility. That is, if you wish to avoid interfering with normal functions of essential links between equipment.