Just take music that you're REALLY familiar with that contains good highs, lows, mids, vocals, some instruments, cymbals and such, and you have yourself a test disc. It doesn't matter so much what you listen to as how familiar you are with it. All the test CDs in the world won't help if you listen to it with no frame of reference. I have a CD with a little of all the above, great soundstaging material, highs, lows, harmony, etc, etc, ad nausium, buy when I listen to something with them I can immediately tell what it does better/worse/as good, etc. since I have a frame of reference. Then I decide which is preferable to the other to help further the decision making process of what I'm going to buy. Sometimes a certain item is "better" but I don't like it for some reason, and since I will be the one listening to it, that's all that matter: whether I like it and will listen to it for hour after hour!!
Basically, this is what Mr PD said, but in a manner a little more verbose!!