It won't be cool if testnet tokens are trading. That would mean in some sense that there is no seriousness in what you are doing because you are still testing your platform and want to launch them on exchanhe. Its not right
I think it depends on the policy of the coin developer, when they want to have a trading volume from a very high tesnet then they can directly register at the exchange with all the concepts and networks ready, or it can be done on a small scale without using a place big exchange.