Fix compliation errors in Testing README sample custom verifier

[MattKotsenas]

Copy / pasting the sample custom verifier from the bottom of the Testing README results in 9 errors and 2 warnings:

Severity	Code	Description
Error	CS0117	'CSharpAnalyzerVerifier.Test' does not contain a definition for 'TestCode'
Error	CS1061	'CSharpAnalyzerVerifier.Test' does not contain a definition for 'ExpectedDiagnostics' and no accessible extension method 'ExpectedDiagnostics' accepting a first argument of type 'CSharpAnalyzerVerifier.Test' could be found (are you missing a using directive or an assembly reference?)
Error	CS1061	'CSharpAnalyzerVerifier.Test' does not contain a definition for 'ExpectedDiagnostics' and no accessible extension method 'ExpectedDiagnostics' accepting a first argument of type 'CSharpAnalyzerVerifier.Test' could be found (are you missing a using directive or an assembly reference?)
Error	CS1061	'CSharpAnalyzerVerifier.Test' does not contain a definition for 'RunAsync' and no accessible extension method 'RunAsync' accepting a first argument of type 'CSharpAnalyzerVerifier.Test' could be found (are you missing a using directive or an assembly reference?)
Error	CS1061	'CSharpAnalyzerVerifier.Test' does not contain a definition for 'RunAsync' and no accessible extension method 'RunAsync' accepting a first argument of type 'CSharpAnalyzerVerifier.Test' could be found (are you missing a using directive or an assembly reference?)
Error	CS0117	'CSharpAnalyzerVerifier.Test' does not contain a definition for 'TestCode'
Warning	CS8625	Cannot convert null literal to non-nullable reference type.
Warning	CS8625	Cannot convert null literal to non-nullable reference type.
Error	CS0080	Constraints are not allowed on non-generic declarations
Error	CS1520	Method must have a return type
Error	CS0305	Using the generic type 'CSharpCodeFixVerifier<TAnalyzer, TCodeFix>' requires 2 type arguments

Which can be addressed with these minor edits:

1. Remove the default null value for Diagnostic, given that the delegated method is also non-nullable
2. Fix the location of the generic params on the nested CSharpCodeFixVerifier.Test type
3. Remove generic constraints on the inner type CSharpCodeFixVerifier.Test (because they belong on the outer type)
4. Fix constructor name on CSharpCodeFixVerifier.Test

[sharwell]

The main issue I have is the discrepancy between the README and the actual code we produce with the new analyzer/code fix project template. The true code is split across several files here:

https://github.com/dotnet/roslyn-sdk/tree/main/src/VisualStudio.Roslyn.SDK/Roslyn.SDK/ProjectTemplates/CSharp/Diagnostic/Test/Verifiers

[MattKotsenas]

The main issue I have is the discrepancy between the README and the actual code we produce with the new analyzer/code fix project template. The true code is split across several files here:

https://github.com/dotnet/roslyn-sdk/tree/main/src/VisualStudio.Roslyn.SDK/Roslyn.SDK/ProjectTemplates/CSharp/Diagnostic/Test/Verifiers

The code we're currently pointing people to doesn't compile, so I'm happy to make any requested changes, but I don't think the status quo is sufficient here.

Edit: Re-read your comment and looked at the template. Edit below:

Now I understand: these helpers are created as part of the Roslyn Analyzer project template. I hadn't seen that before because I was working on an existing analyzer project, and hence didn't go through the template flow.

In that case, does it make sense to remove this section of the README entirely, and instead point to the code with an explanation that these are provided as part of the template?

[MattKotsenas]

OK, replaced the snippet entirely with a link to the template and a link to instructions on instantiating the template using VS instead.

I don't love this solution because it's another layer of indirection and relies on devs either parsing the template by hand or using Visual Studio, but it addresses the potential for drift between docs.

@sharwell, mind taking another look? Thanks!