A crappy test is about as easy to make as a crappy level. A good test is about as hard to make as a good level. Having a good progression of difficulty, an original concept (or an original take on an old concept), plus the patience to test the whole thing takes a lot of effort. If I had to compare the process to anything, I would say it's closest to making a single player level, or perhaps a race level. You also need to be really good at test levels in general, since if you make a level that you haven't run through extensively and tested, it really shows.

Of course, with these levels in general, you either love them or hate them. If you hate them, all test levels will seem like crap. If you love them... well, they still LOOK like crap.