You shouldn't need a wire any heavier than #12 AWG. If the wire is used in 'free air' you can use a 30-amp fuse. If the wire is under a carpet or bundled with other conductors the fuse size should be no larger than 20-amp because the insulation cannot shed heat (like it can in free air).
The test light shown is NOT a good one for several reasons:
It doesn't present a load,
No voltage range is advertised,
LEDs light in the handle when voltage is present.
Some of my best troubleshooting tools are ones I made from parts hanging around. I have even sacrificed test leads because I needed the wire or the clips, etc. Anyone can always run out and buy expensive test equipment that doesn't work any better than ones you make.
Continuity lights are a good example. Ideal sells a chrome model but the cheap plastic ones are safer to use around hot wires and injury from shock is nonexistent:
A continuity light is simply a flashlight bulb in series with a AA battery and it is used on 'dead' wires (for checking fuses, motor leads, switches or ringing out (identifying) wires in a bundle). Sometimes we use a continuity light WITH a jumper wire for circuits with several switches in series, especially when the circuit has more than one faulty switch.
Meters and LED test lights give false readings because they do not 'load' the circuit and are too sensitive. A larger incandescent lamp works best especially for checking connections. - Dave