I have found a work around for this problem. I created a Portable Class library project and moved the non test class code to this project. I created a new Test project for the desktop and moved my test classes to that project and reference the class library project.
As a result this all works good enough now for me to continue with my projects.
Note: I tested this by including a Universal Windows 10 App project and I reference the class library. I also created a new Universal Windows 10 Test project and linked the files from the desktop unit test project, and it all works.
I did have to change many internal and private methods and classes to public, or I could have added an assembly directive to treat the test project assembly as a friend of the class library. I would then have to link the class library files to the App project instead of adding a reference only, to get it to work and not change my access levels.