I have a c# solution with the following structure:
mySolution
myProject
myProject.MSTests
References
Microsoft.VisualStudio.QualityTools.UnitTe
Had this same issue but reading over the previous answers, everything looked good.
In my case, I had just run the test suite made a small change, built the solution and tried to run the test. No go. I tried building a couple more times and looking for problems other people had tried. Still no go.
I hit enter in one of my test methods to add a new and hit F6 to build the solution and clicked run Unit Tests.
Bingo! Everything ran smoothly.