Use specific constraint

[manfred-brands]

Fixes #824
Fixes #826
Fixes #828

@mikkelbu The first commit is not directly related but was necessary to allow NUnit 4 specific names to be only tested when building against NUnit instead of making up the names in the analyzer code.

The second commit add the new analyzer and code fix.

There was a small change in the documentation tests as they were not testing for the new Style analyzers.

If you think this analyzer should be in a different category, that is fine by me.

[mikkelbu]

Looks great @manfred-brands and I think it is fine to use the 4XX category for this (at least I don't have any strong opinion about this).

I've added a couple of suggestions, but that is all

[mikkelbu]

I forgot a question or two - mostly just curious, if I overlooked something.

It there a reason for NUnitFrameworkConstants and NUnitLegacyFrameworkConstants being abstract and NUnitV4FrameworkConstants is not?

Also is there a difference between abstract and static in this case - i.e. the classes only containing public const string... properties and no inheritance (NUnitFrameworkConstants was static before, but it was changed to abstract).

[manfred-brands]

I forgot a question or two - mostly just curious, if I overlooked something.

It there a reason for NUnitFrameworkConstants and NUnitLegacyFrameworkConstants being abstract and NUnitV4FrameworkConstants is not?

Also is there a difference between abstract and static in this case - i.e. the classes only containing public const string... properties and no inheritance (NUnitFrameworkConstants was static before, but it was changed to abstract).

I have no access to a PC until next week, but from what I remember there was an error at one time that I could not use a static class somewhere, although I don't think I used inheritance. It might no longer be the case. I'll check when I get back.

The main difference between static and abstract here would be inheritance.