I'd like to first of all give my "thank you" to this blog post by Jason Kemp: http://www.ageektrapped.com/blog/unable-to-set-testcontext-property-for-my-class-net-compact-framework/ which saved me from going crazy.
It often happens when you are playing with your CS project's references in Visual Studio and you end up doing something (not necessarily stupid things!) and mess up the entire project. I had the same experience today, and I've done everything I can to recover it. However, I was still trapped by this problem of "unable to set testcontext property for the class".
And the solution?
It's that easy: restart the Visual Studio, and it's gone.