Would it make sense to test the performance and handling of a prototype aircraft by mounting a free moving mock up on a track (like a rocket sled)?
No, it doesn't make sense because there are better alternatives:


Computer models: fluid dynamics programs are very good at simulating airflow over a 3d model
Wind tunnel testing: In a wind tunnel the airplane model stays in the same place and air is blown over it. Smoke can be added to show vortices, and cameras can record it


There are practical problems with rocket sleds:


A rocket sled would be much harder to get video from
It would be challenging to keep a rocket sled at a completely consistent speed
Rockets only last a few seconds, you'd only get a bit of data at a time