This article provides an article on how to create unit tests for a Visual C# class in a Windows Store app with Visual Studio Express 2012 for Windows 8 and the Microsoft Unit Testing Framework.
This unit testing process described in this article follows a Test-Driven Development approach: you start by writing a test method that verifies a specific behaviour in the application that you want to test and then you write the code that passes the test. This article also suggests to create a single Visual Studio solution and separate projects for the unit tests and the DLL that you want to test. You can also include the unit tests directly in the DLL project or you can create separate solutions for the unit tests and the DLL.
Read the complete article on http://msdn.microsoft.com/en-us/library/windows/apps/jj159318.aspx