SourceGenerator test

[WeihanLi]

Maybe we could use Microsoft.CodeAnalysis.CSharp.SourceGenerators.Testing.XUnit for generator unit testing

• https://github.com/dotnet/roslyn/blob/main/docs/features/source-generators.cookbook.md#unit-testing-of-generators
• https://github.com/dotnet/roslyn-sdk/tree/main/src/Microsoft.CodeAnalysis.Testing
• https://www.nuget.org/packages/Microsoft.CodeAnalysis.CSharp.SourceGenerators.Testing.XUnit/

[IEvangelist]

I like that idea, let's make it so. It should help us to discover issues sooner, for example; when I updated the generator to .NET 6 from .NET Standard 2.0 - I didn't realize that would break the core functionality.

[IEvangelist]

We have tests, closing this...