Investigate SDK test failures and finalize "enable producing reference assemblies by default"

[rokonec]

We have reverted #8111.
See #8131 for more details.
Related failing SDK insertion PR was dotnet/sdk#28869

We shall either fix in msbuild or find out how to fix test in SDK.

[drewnoakes]

This has been fixed.

[rainersigwald]

Note that I've recieved a couple of offline reports of trouble from this that make me suspect we'll have to revert again, but we're not quite there yet.