I had planned on making this much earlier than now, but as you can see from the photos above, the first attempt did not work out very well. The wood (balsa) I used for it was much too soft, so precision was difficult, I used old wires and back then my soldering skills were terrible, so there were definitely some loose connections, or it was being short-circuited (it was dead within two hours), and I didn't have a battery holder, so I tried to connect two AAA batteries with tape and tin foil (didn't work out very well).
However, in this version I fixed all of those problems and improved the overall design. My new design was inspired by Instructables author Brennn10 and diyworkbench's LED testers.
Rather than making only one set of testing terminals for one LED at a time, I made several terminals, so many different types and kinds of LEDs can be tested easily. Some might have very short legs, for example. For that the large plates would be useful. Or you might want to compare some different LEDs, to see which one has brighter colors, for instance. Then you could plug them all into the female sockets next to each other. I also added terminals with a lower current, as some LEDs will burn out with only 100 Ohms. A 3.3k Ohm resistor did the job well.