Fix namespaces in Nitrox.Test to match folder structure

[Measurity]

Why?

• Resharper warns to adjust namespaces if they don't match the folder structure. There's even a refactoring option to adjust the namespaces of an entire project to match the folders. If we applied this refactoring now, it would break our namespace convention in Nitrox.Test.
• There doesn't seem to be an option do disable the resharper namespace check on a per project basis.

When?

Since renaming folders effectively moves the code files this change requires planning to not cause merge-conflicts for other developers. It should be done closer to a release (candidate) of Nitrox.

How to fix

1. Set root namespace of Nitrox.Test to empty:
   
2. Rename the folders to match the project namespaces it tests: Model -> NitroxModel