roslyn: SDK 'Microsoft.NET.Sdk' not found loading CSharp project via MSBuildWorkspace.OpenProjectAsync

In the 15.9 generation of build assemblies and so forth.

We think Roslyn should be emitting some diagnostics, but more than likely we are either not handling a Solution or Project correctly, or perhaps have not fed Roslyn with the correct options, properties, etc, along these lines.

We cooked up some unit tests here for demonstration and verification purposes if someone could take a look.

Unit tests are appropriately named, one should be producing obvious errors. The other one may or may not, but in its current form we cannot know for certain.