But if they disconnect them when the car is running and don't reconnect them to the battery (I may be overreacting in stating it could be very dangerous to reconnect battery cables while the car is running as I don't know if there's a specific way to do it and not get electrocuted/shocked) then they run those risks of frying the car's electronics.
Probably safter to just do the test, then turn off the car and then reconnect the battery (or the new "good" battery).