A lot of people struggle with relative path in DeploymentItem. The relative path is from Solution Folder. This can only apply when run from Visual Studio, because mstest.exe is launched with compiled project (and not sln file) as dll and thus it cannot correctly determined the relative path of deployment item.
I have found that I should set the DeploymentItem relative path in testrun.config by RelativePathRoot element. But there is almost no documentation for RelativePathRoot which is strange. How do you set relative path to MSTest?