It is possible to set up an account without a linked bank account, but I think an easier way to do what you want to do is simply set up another Location under your current account and use that for training. That's what we did when we first switched to Square and it worked well for us.
If you're not familiar with Square's Location facility, see this article which thoroughly explains how to use separate Locations under the same account.
Judy T Shumway
