I did the same thing that John Gardner showed in his answer, except that I divided it into 2 projects (to imitate the setting that I worked with in my real solution). This did not work at first, although I could see the tests in Test Explorer. After a short search, I came across the correct answer for my situation.
Turns out you need:
(1) Create a new Unit Test project (and not a class library)
(2) Add the NUnit link to the Unit Test project (I used NuGet) and installed the NUnit test adapter in VS2013
(3) Transfer your tests to this new project
(4) After saving and assembling, now you can return to your production code and see the message "x / y transmission" and view the unit tests in the Test Explorer window.
Most of us who have been using NUnit for a long time are used to create class libraries for our code instead of Unit Test Projects. It would be nice if the CodeLens documentation directly touched on this (the documentation reads: "Test status indicators appear automatically in test projects," which was my hint).
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