Add reference assemblies for net6.0-[platform] TFMs

[Youssef1313]

https://github.com/dotnet/designs/blob/main/accepted/2021/net6.0-tfms/net6.0-tfms.md#design

[Youssef1313]

@sharwell Is this possible? If yes, it would be nice to have :)

[jmarolf]

So the ask is to have Net60Windows, Net60Android, etc in addition to Net60 like we have here?

roslyn-sdk/src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/ReferenceAssemblies.cs

Line 887 in b7df823

public static ReferenceAssemblies Net60 => _lazyNet60.Value;

I have no problem with that, but it would require there to be nuget packages with these references. Currently we rely on Microsoft.NETCore.App.Ref.

This is my best understanding of the reference nuget packages and how they map. You will need to add all the reference from Microsoft.NETCore.App.Ref as well.

Property Name	Nuget Package
Net60Windows	Microsoft.WindowsDesktop.App.Ref
Net60Android	Microsoft.Android.Ref
Net60iOS	Microsoft.iOS.Ref
Net60MacOS	Microsoft.macOS.Ref
Net60MacCatalyst	Microsoft.MacCatalyst.Ref
Net60TvOS	Microsoft.tvOS.Ref
Net60Tizen	???

[Youssef1313]

@jmarolf Yes that's exactly what I'm asking for.
I'm not sure whether there are NuGet packages for these or not. But the lack of these reference assemblies makes testing analyzers/generators that are very domain-specific in these areas nearly impossible.

[Youssef1313]

@jmarolf Thanks!! That was too fast :)