This might be out of date now, but this is how I have it setup and working - downvote and let me know if it's wrong and I'll delete this. I got it from a blog post/MSDN page, but I can't locate it any more.
First you need to create a TFVC Team Project (doesn't matter if you don't use it again).
Into $/MyTFVC/BuildProcessTemplate/CustomActivities/
Checking the following files from xunit.net:
Now in VS, click the BUILD, Manage Build Controllers... option. Select the "Hosted Build Controller (Hosted)" and click "Properties...".
Enter the path where you checked in the DLL's into the "Version control path to custom assemblies" field:
You should be good to go.