Add unit and integration test projects

[JSkimming]

@tracker086 I'd like to start adding unit tests but before introducing them I just wanted to check if you have a preference on Framework?

My personal preference is to use a combination of xUnit for functional testing and Machine Specificatons contextual and behavioural testing.

What do you think?

[adrifer]

I usually use the Visual Studio unit testing framework. But I don't care to use a new one and learn. So it's fine to me to use xUnit

[JSkimming]

👍 The MSTest test framework while fine, can be limited, it's why some Microsoft People created xUnit, and I understand xUnit much more widely used within Microsoft than the MSTest.

I'll get on with adding some tests over the coming days, there's a lot of code in the UserStore so it would be good to get the coverage up there.