I would posit that reasoning is the ability to construct new, previously-unexpressed information from prior information. If ChatGPT existed 110 years ago and fed all the then-known relevant experimental data regarding subatomic particles, it would not have been able to arrive at the new notion of quantum mechanics. If it existed in the medieval era and was trained on the output and influence of Galen, it would not be able to advance beyond the theory of humours to create germ theory.
It's only because quantum mechanics is a known concept that has been talked about in literature that ChatGPT is able to connect that concept to other ones (physics, the biography of Niels Bohr, whatever).
So the test for actual reasoning would be a test of the ability to generate new knowledge.