How to convert an existing assembly to a ms unit test assembly?

The problem is that test projects are "marked" on the project file - you can convert a class library to Test project follow these four simple steps:

1. Unload the project (.prj) file and then open it for update.

2. add the following line to the project
   C#:

   <Project>
    <PropertyGroup>
     <AssemblyName>....</AssemblyName>
     <!-- add this line below -->
     <ProjectTypeGuids>{3AC096D0-A1C2-E12C-1390-A8335801FDAB};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
    </PropertyGroup>
   </Project>
   

   VB - <ProjectTypeGuids>{3AC096D0-A1C2-E12C-1390-A8335801FDAB};{F184B08F-C81C-45F6-A57F- 5ABD9991F28F}</ProjectTypeGuids>

3. Re-load the project back
4. Run you (now working) tests

Note that you'll need to manually add reference to Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll in order to be able to use test related attributes

Update: In the upcoming MSTest V2 this will not be nesessery as MSTest becomes a NuGet package which works just like NUnit/XUnit