The WorldPay test URL is currently set as "secure-test.wp3.rbsworldpay.com" it is failing on testing, WorldPay have advised me that the URL should now actually be "secure-test.worldpay.com/wcc/purchase
The source here is correct - the GetWorldpayUrl() method in the Nop.Plugin.Payments.WorldPay.WorldPayPaymentProcessor class returns the test and live URLs as per your issue.
The WorldPay test URL is currently set as "secure-test.wp3.rbsworldpay.com" it is failing on testing, WorldPay have advised me that the URL should now actually be "secure-test.worldpay.com/wcc/purchase