Be sure to log in to your payment gateway account to verify that payment was received.
Payment testing is advisable to ensure that your payment gateway is correctly configured and can accept live payments. If there is a problem, it's preferable that you discover it before one of your entrants.
Testing in test mode, with test/dummy credit card details, is not adequate— we have seen cases of test mode working fine but real payments failing.
If you have direct control over your payment gateway account, you can refund the test entry fee after a successful payment.
Alternatively, you can temporarily set the entry to e.g. 50 cents and complete an entry paying this test price or create a discount code. Don't forget to set the entry fee back to full price afterwards though!