MSTest is a command line utility from Microsoft that executes unit tests created in Visual Studio 2005, 2008, 2010 and is still available for backwards compatibility with older versions in Visual Studio 2012 and 2013. Microsoft recommends running the tests in VSTest.Console in later versions.
MSTest is not the Visual Studio Unit Testing Framework defined in the Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll dynamic link library (DLL).
References
MSTest Wikipedia(Text) CC BY-SA