What is a successful usability test?

When a design has a usability problem, it is because someone made a wrong decision. They chose to take the design in a direction that creates frustration for the user. A different design choice would have prevented the frustration.
There are two outcomes from poor decisions: either the user experience is worsened because of a change that just shouldn't have happened; or a valuable opportunity is missed to improve the design's user experience. Either way, when usability tests work, these results are significantly less likely.
I consider a usability test to be successful when the design team members receive the information they need to make the right decision. Successful usability tests produce informed decisions.