Tina - I think you have asked a very interesting an important question, and I don't think you have got the answer such a good question deserves.
I think that the 'test is the exercise, and the exercise is the test'. Let me explain. When we do repetitive exercise like running and walking, we develop structural imbalances (eg muscle shortening and joint stiffness) that we need to keep in check. This exercise / test helps us check for pelvic (core) shortening, weakness and asymmetry. Hope you find this idea and explanation helpful. Regards Martin