Hi yshvrdhn,
As far as I know, The Visual Studio UI Test Plugin for Silverlight is just released for VS2013. And I don't get any information about the version for Visual Studio 2015 at present.
I suggest you use VS2013 and the
Visual Studio UI Test Plugin for Silverlight for VS2013 to implement the Code UI Test for Silverlight until the version for VS2015 release.
Best Regards,
Weiwei