You have to run init before update:
git submodule init
git submodule update
After that to run the integration tests for git-tfs you will need to get XUnit up and running:
https://github.com/git-tfs/git-tfs/wiki/Running-the-Unit-Tests
XUnit test runner for VS2012: http://visualstudiogallery.msdn.microsoft.com/463c5987-f82b-46c8-a97e-b1cde42b9099
To run the integration tests then you will have to install the TFS plugins (Team Explorer) for VS2008, 2010 and 2012. But you can just unload those projects (GitTfs.Vs2008, GitTfs.Vs2010 and GitTfs.Vs11) if you don't want to run them. GitTfs.VSFake is a fake for the different Visual Studio versions and that will let you run most of the tests.