Add support for DiagnosticSuppressors

[manfred-brands]

Fixes #165

I opted for the same set of overloads as for RoslynAssert.Diagnostics, but are open for change there.

Priniciple is:

• Check the code for '↓' or use ExpectedDiagnostics
• Compile code without DiagnoticSuppressor to ensure that ExpectedDiagnostic is raised.
• Compile code with the DiagnosticSuppressor and check if the diagnostic IsSuppressed.

I tried it out with one of my analyzers and it seems to work fine. Once this is in and release, I will update nunit.analyzers to use it as well, dropping the code there.

Initially I went for different approach, but the 2nd version aligns much better with the existing framework. I have not looked at the analyzer to see what needs changing there so that it warns of missing '↓'. It doesn't do so at the moment.

Fixes #253

Not related and we can split this into two PRs is you prefer.

[JohanLarsson]

Very nice, thanks! I'll try to find time to release a new nuget today.

[JohanLarsson]

https://www.nuget.org/packages/Gu.Roslyn.Asserts/4.1.0

[manfred-brands]

Thanks