I didn't find many developer resources regarding the package Episerver.Marketing.Testing but when I installed on an Alloy-site I had A/B testing working there. Though when I installed the same package on the site I'm working on, the A/B test changes option is disabled. What could cause that?
My site: https://goo.gl/photos/jwDFXe1GFWprsYGp8
In my case (with custom authentication implemented) the logged in user had to be part of the role CmsAdmins and WebAdmins to enable the option.