Any old sets that are no longer of any use on the tree make brilliant "test lamps"!
I've got a set that were about to go in the bin, but instead yielded a nice size wire for testing my FBH (Fuel burning heater.) firing as it was just right to fit in the hole for Pin 3. The other end gave me a good three feet of cable with a bulb on the end for further tests, and what's more, I have around 20 spare bulbs for future use!
(and should be more than safe for 12v as this was an older set that was mains voltage)