This can occur if you have had your event customized or you have implemented a custom page into the donation or registration flow. The custom piece may be taking inadvertently redirecting you out of the Test Drive workflow and resolving to live links which is why the test credit card is failing to process.

All test drive links include reference to "Preview" in the URLs that are generated. You can verify whether a page is a test drive page by reviewing the content of the URL to check for this parameter. If it is not included, then you are most likely on a live, front end page.