Add UI-Tests / Unit-Tests

[SeriousM]

The project started long time ago and all we needed was a test application to see if the localization works.
Since then the library evolved to an "almost product" and people complaining that "this or that" doesn't work. We, @MrCircuit and @SeriousM, don't have the time to always investigate these errors and therefore I propose to create a real test suite that covers all critical and optional scenarios.

Based on

• nUnit for backend tests
• White for UI tests

Providing this tool, users can add their own tests (mandatory for further enhancements!) and we can verify easily if something is broken.

[xmedeko]

I recommend xUnit for backend testing.
And the White project has different home now.

[konne]

White is in my eyes a dead project. So will add
https://github.com/Microsoft/WinAppDriver
as new possibility for GUI Tests