Add a package test that uses nunit framework 4.0

[CharliePoole]

That way we'll know when and if the api changes!

[CharliePoole]

@manfred-brands Would you be interested in doing this one?

[manfred-brands]

I don't mind trying. I have to make myself familiar with your tests first.

[CharliePoole]

OK, you've got it... here's an outline of where I think you would need to start.

1. Create a target Assembly. I suggest copying the mock-assembly project, making some name adjustment and using the latest NUnit 4 as the framework.
2. Create some new tests. The tests are defined in package-tests.cake. We should have at least one targeting .net fx and one core or more... up to whatever targets nunit4 supports.
3. There are lists of tests in package-definitiions.cake. You'll want to add them to the standard runner tests and the net core ones to the netcore tests as well.
4. Run them using build -t Package or ./build.sh -t Package on linux.

Hope that helps.