Rebrand `unittest` project to `xunit`

[peterblazejewicz]

The RC contains 2 different flavours of test framework: MSTest and xUnit:

Templates                            Short Name       Language      Tags          
----------------------------------------------------------------------------------
Console Application                  console          [C#], F#      Common/Console
Class library                        classlib         [C#], F#      Common/Library
Unit Test Project                    mstest           [C#], F#      Test/MSTest   
xUnit Test Project                   xunit            [C#], F#      Test/xUnit    
Empty ASP.NET Web Application        web              [C#]          Web/Empty     
MVC ASP.NET Web Application          mvc              [C#], F#      Web/MVC       
Web API ASP.NET Web Application      webapi           [C#]          Web/WebAPI    
Nuget Config                         nugetconfig                    Config  

The generator supports only unittest, so first migrate unittext to xunit, merge into vs2017-rc branch and only after this migrate (now) xunit project to .cproj and add mstest project.

Thanks!

[peterblazejewicz]

closed via: 4349436