Use url_for to generate URLs

> it is about consistency and updates, if that endpoint changes in the future
> we don't have to update hundreds of tests for a specific string. The actual
> url should be ambiguous we are testing a view endpoint.

tests/app/main/views/test_two_factor.py

@@ -28,7 +28,7 @@ def test_should_login_user_and_redirect_to_dashboard(notifications_admin, notifi
                                    data={'sms_code': '12345'})
 
             assert response.status_code == 302
-            assert response.location == 'http://localhost/services'
+            assert response.location == url_for('main.choose_service', _external=True)
 
 
 def test_should_return_200_with_sms_code_error_when_sms_code_is_wrong(notifications_admin,