Disclaimer: it is not about xunit with visual studio 2015, but Visual Studio 2017 with a UWP unit test application (MSTest). I got to this thread searching the same thing so maybe someone else will do the same :)
The solution for me was to update the nuget packages for MSTest.TestAdapter and MSTest.TestFramework. It seems that when you create a unit test app for UWP you don't automatically get the latest versions.
