A song I always like to use to test headphones is Machine No. 2 By Leo Kottke. The version from Mudlark (1971). It's good for a lot of reasons. Firstly, I know the song really well, and secondly it has everything you'd want to test headphones on. The bass is rapid and has many runs up and down the fretboard, there's a lot of fast intricate fingerpicking to test the higher registers, and the drums are simple but integral (mainly bass and snare), AND the recording from mudlark has some good stereo implementation. It's also quite a good recording. It's basically my first stop when I try some new headphones.
This is youtube, so it's not gonna sound too good, but here's a link...
https://www.youtube.com/watch?v=X2Sh8nfzR-w
And along the same lines, but with vocals as well (and some incredible double bass from Danny Thompson) Springtime Promises or Light Flight by Pentangle are good choices...
Mind you, these are all folksy tunes...