Yep, it's built in to watch and check for the live key on purpose. We've talked about allowing test keys but it creates foot-guns where the user might accidentally keep test mode on after taking bookings. Then, because test mode is on, the booking money would not actually flow to your account. So we've stopped it from doing that.
So how to test? We recommend using your real credit card to do a booking. Create a quote and send it to yourself as a pretend guest, open it online, read every page as you proceed, see what needs to be changed and then actually book it. After booking, go into the booking on the control panel side > Transactions > Payment > Void and Delete to stop the payment from settling. Then cancel the booking.
