If you are testing with a Salesforce Developer account this is similar to a live Production environment and allows you to test both live and test transactions. When setting up your gateway be sure to specify whether you are setting up a test or live gateway using the correct credentials for each.
Test gateways will automatically have the Test Endpoint box checked and automatically send to a test endpoint for your gateway. Live gateway setup will leave this box unchecked and will automatically send the transaction to your live environment.