Test of what...an ability to play Alberti bass?
Personally, i have no great love of his sonatas and find them predictable. Perhaps i have only ever listened to poor recordings.
Mozart, it could be argued, is the ultimate test. Mozart requires light, mobile, highly reactive fingers. He requires exceptional talents of articulation and the ability to produce a highly nuanced range of sounds. There are no crutches, such as a thick texture to hide the single notes, distracting virtuoso figurations and the possibility to rely on pedal.