Today I have stumbled onto an interesting problem: I had unit tests for my Node.JS project in Project A, but after I have moved the file (simple drag&drop in Visual Studio) to Project B, the unit tests were no longer showing up in the Test Explorer window.
There were no "build" errors, nothing, Visual Studio just wasn't discovering them anymore.
So and it has turned out that after the Drag&Drop operation one of the file properties was not preserved: TestFramework. In my case I just had to set it to ExportRunner.
Changing that did the trick :)