UX (User Experience) is the process of creating and building software products (web/app/etc) that provide relevant experiences to users. It includes aspects of branding, design (UI), usability and function. In simple, usuability is subset of UX.
So when you do Usability testing, it’s only checking if product is easy to use with real users.
For UX testing, you must care more aspects such as pleasure of real users, efficiency of functions, GUI, etc.