Test credit cards (found at https://docs.recurly.com/docs/test) will only work on a site that is in sandbox mode. We do not have any test cards for production sites. In Production mode your site connects directly to your gateway's production environment for processing live payments.
In a production scenario when a transaction must be tested, we often suggest using a regular card, and simply voiding the transaction as soon as it succeeds. This will allow you to confirm if your payment gateway is configured correctly, but will not actually settle the charge and take money. If a "pending" transaction shows up on a statement from that test, it will drop off within a short time typically 2-7 business days.
For ongoing testing please consider creating a second site that remains in sandbox mode.