Extend AnalyzerTest to support tests for DiagnosticSuppressor analyzers

[tom-englert]

This implements scenario 1 from #915 by conditionally selecting dependencies and extend the analyzer validation code.

[sharwell]

❗ These can't change. Light-up/reflection must be used to support the scenario in cases where the APIs do not exist.

[tom-englert]

OK, then I will do another PR using reflection.

[sharwell]

We have a few places where it's already used. See CodeActionExtensions for an example:

roslyn-sdk/src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/Extensions/CodeActionExtensions.cs

Lines 15 to 24 in 27fe3d6

	public static ImmutableArray<CodeAction> GetNestedActions(this CodeAction action)
	{
	var property = typeof(CodeAction).GetTypeInfo().DeclaredProperties.SingleOrDefault(property => property.Name == "NestedCodeActions");
	if (property is null)
	{
	return ImmutableArray<CodeAction>.Empty;
	}
	return (ImmutableArray<CodeAction>)(property.GetValue(action) ?? throw new NotSupportedException());
	}

[sharwell]

Also the support for AnalyzerConfigDocuments #734

[sharwell]

💡 These don't need to be added to the solution. They are visible in Solution Explorer with this button:

[tom-englert]

It's not about viewing the file, it's about getting FindInFiles with scope solution working as expected. (for guys like me being not so used to all the features and tweaks in an unknown solution)
However I won't insist in adding this...

[sharwell]

I find it's a losing battle to keep solution files synchronized with the solution. The only way it's always consistent is to omit them all and point users to the solution filter button. 😄

The button used to work much better in Visual Studio 2017 (one click would toggle show/hide). Now it's tedious but still technically works.

[tom-englert]

Replaced by #917