This is a project that uses T4 templates to generate a MsTest wrappers for any nUnit tests found in the solution.
It was created to allow unit test, that were written using nUnit, to be exposed in a manner that allowed them to be used in Microsoft Test Manager based test automation.
Beyond the actual T4 template, which you may need to edit to meet your needs, all the interesting work is done in the CSPROJ file. In this file there are edits (around line 100) were targets are added to allow the template based code to be regenerated on any re-compile of the solution (and to expose MSBuild properties). This is as opposed to the re-generation only occuring when the .TT file is edited.
Add the GeneratedMstests project to your solution and add reference from it to any projects in the solution containing nUnit tests.
Extra background and documentation can be found in this blog post