Cannot start test project “xxxx” because the project does not contains any tests

This error happen when we try to launch a Visual Studio Project defined as test with another testing framework ( ie NUnit ) and we remove all the visual studio test from inside it. To me happened when I try to revert back to NUnit a project with standard Visual Studio tests. Here The message:

The solution is to open the project ( csproj ) file with a text editor, and look for the lines below:

By removing the line in evidence we go back to a standard class library project and the problem is solved.