Using these keys below, you can do your tests on localhost :

Site key: 6LeIxAcTAAAAAJcZVRqyHh71UMIEGNQ_MXjiZKhI
Secret key: 6LeIxAcTAAAAAGG-vFI1TnRWxMZNFuojJ4WifJWe

Google add a message on recaptcha to prevent users and the administrator that keys are just for tests.