You can use several methods to make test Registrations & Payments as outlined below.
To test new credit card merchant accounts or new Paypal credit card transactions it is recommended to use options 3 or 5.

1. Make a booking using Add Registration (Manage Tab) and don’t select the Credit Card  radio button (as for sponsors). 
2. Enter a valid cc number but use an invalid exp date and ccv number. 
3. Set up a temporary reg type for an amount of say $0.50. This way you can do a real payment transaction incurring just a minimal charge. This method is recommended when you first set up your merchant account. By making a real payment you can check to see the payment appears  in your merchant account or payment gateway and that the payment appears on your credit card statement (plus you can see how it would appear on your Attendee’s credit card statement. 
4. Use  test Visa credit card number 4111111111111111  to test registrations. This is a nearly universally recognized test number for a Visa card and will obviously NEVER result in an actual transaction being completed.  Most gateways support this such (for example including  IIRC, Camtech, St.Goerge  and BNZ ). Please note that the cents amount of the transaction may affect the returned response (for example  St.George and BNZ). That is $25.00  would test as approved whereas $25.25 would return an error code of 25 from the processing payment gateway resulting in a declined transaction. This is by design to be able to test various failures. Invoices will be marked as preview/test and will be recorded against the project as a Test mode registration (Project Status: Test Mode).
5. Add a temporary Discount Code in Setup step 6.  Set the Discount Code to a percentage or fixed amount that would allow you to pay a minimum (eg 50c). This method is also recommended when you first set up your merchant account.This method is a ideal if your event is live as only you will know the code to enter. Once tested you could then delete the discount code.