I have one of them things, Ill sell it for $101 right now, lol...
I still use a test lights, I have like 10 of them, especially on the trailers...
I still have some I made myself, I have one I made out of some curly cords and a door buzzer, it has 2 wires, you put the buzzer in your pocket can feel when you compllete the circuit. I also made myself a contuity tester, not sure if I still have that, but it was simple, a 9v battery another buzzer and 2 leads, great for testing fuses with the power off...