I find it somewhat interesting that no one else has asked this question. The purpose of the device is for safely bypassing a photocell. IMO, the safest way now would be to deenergize the circuit, replace the photocell with a test cap, and then reenergize the circuit. For a single person, this can be quite time consuming considering the trips from the panel board up to the luminaire. I had a situation in a parking lot where many ballasts were burned out and in the shorted state. I think this may have led to the photocells also being defective. Before opening the luminaire for inspection, I attempted to replace the photocell with a new one. Because of the shorted ballast, the twist-lock plug and the receptacle did not play nicely when contact was made.
Ideally, a testing device would be made to withstand a fault. Perhaps a momentary switch would control a latched relay only after the cap is locked in place. The device would have a breaker for easy reset.