To deal with accidental or practice exams there are 2 basic parts, one for the VetBadger encounter and another for the QuickBooks invoice.
On the VetBadger side, you can mark the appointment as "invalid" by going to the History page and clicking on the little checkbox just to the left of the appointment description.
For QuickBooks you can either delete the invoice or just mark it VOID, either option should remove the open balance on the account.