Add NUnit.Analyzer to our test projects

[manfred-brands]

Mainly as it is needed for #3376 we should add the NUnit.Analyzers to our test projects.

[manfred-brands]

Adding this resulted in a lot (1k+) analyzer errors/warnings, meaning we have a mismatch between the two.
Some are because the tests are explicitly for those cases, such as calling Assert with mismatched types, calling Assert with constants.

Some are limitations in the analyzer:

• (TypeParameterUsedWithTestMethod<T>) and equivalents. The analyzer would have to deduce the type from the TestFixture attribute. Not sure how often this is used in real life if it warrants the work needed in the analyzer.
• ConstraintTestBase references a property in a TestCaseSource which is not defined in the class, but assumes that a derived class would have one. As the member must be static we cannot specify this as an abstract property. The derived classes get standard .net analyzer warnings about unused fields. Not sure how to fix that nicely.

[jnm2]

I'd usually use pragmas with explanations in a comment on the pragma line.

This doesn't help earlier than .NET 7, but interfaces can declare static abstract members and I wonder if implementing such an interface would cause the compiler to stop calling then fields unused.

[manfred-brands]

@jnm2 Unfortunately the interface approach doesn't help with a derived class hierarchy.
If we put the interface on the base class we need to implement it them in the base class and we can't implement static members as abstract there.