I had same symptoms, but under different circumstances.
I had to add one additional step to Peter Lamberg’s solution — Clean your solution/project.
My unittest project targets x64. When I created the project it was originally targeting x86.
After switching to x64 all my unit tests disappeared.
I had to go to the Test Menu -> Test Setting -Default Processor Architecture -> x64.
They still didn’t show up.
Did a build.
Still didn’t show up.
Finally did a Clean
Then they showed up.
I find Clean Solution and Clean to be quite useful at getting the solutions to play ball when setting have changed. Sometimes I have to go to the extreme and delete the obj and bin directories and do a rebuild.