I have a c# solution with the following structure:
mySolution myProject myProject.MSTests References Microsoft.VisualStudio.QualityTools.UnitTe
The original poster did do this, but I arrived here after not having done this:
Be sure that [TestClass] is declared at the top, public in scope:
[TestClass]
namespace XYZ.API.Repository.Tests { [TestClass()] public class ClientTests {