This repository contains an ASP.NET Core solution demonstrating how you can implement custom authorization handlers using the new Authorization story in ASP.NET Core.
The solution contains a webapp using MVC and a Test project using xUnit. The test project shows how you can unit test your authorization handlers.
You can read some more details in my blog post on http://blog.stoverud.no/posts/how-to-unit-test-asp-net-core-authorizationhandler/.