Explicitly auth against v3 in v3 tests.

Skip v3 tests altogether if no USERID is set.
diff --git a/acceptance/openstack/identity/v3/endpoint_test.go b/acceptance/openstack/identity/v3/endpoint_test.go
index 8b302cc..5f166c4 100644
--- a/acceptance/openstack/identity/v3/endpoint_test.go
+++ b/acceptance/openstack/identity/v3/endpoint_test.go
@@ -13,6 +13,9 @@
 func TestListEndpoints(t *testing.T) {
 	// Create a service client.
 	serviceClient := createAuthenticatedClient(t)
+	if serviceClient == nil {
+		return
+	}
 
 	// Use the service to list all available endpoints.
 	results, err := endpoints3.List(serviceClient, endpoints3.ListOpts{})