Make Microsoft.NET.Test.Sdk package transitive

[Evangelink]

Fixes #3227

I have tested locally with the project linked in the issue that wasn't working before and is now working.

I have also checked that the testhost and other relevant files are correctly copied to the output.

Hopefully there is no unanticipated bad behaviour.

[Haplois]

Please check compatibility with 16.11 version of VS too.

[Evangelink]

It works well

[jonpryor]

This change causes #4098; for certain projects, it means that simply updating the Microsoft.NET.Test.Sdk can cause the project to stop building.

Those "certain projects" are projects which provide their own Main() method and reference Microsoft.NET.Test.Sdk indirectly via transitive dependencies.

Should this be considered a "breaking" change, insofar as updating the Microsoft.NET.Test.Sdk package can cause build failures?

What is the best way to resolve this? Document this change somewhere? (NuGet.org doesn't have a link for "release notes", so I'm not sure how anyone would find any such documentation, but it's a least a plausible answer?)

Make inclusion of e.g. Microsoft.NET.Test.Sdk.Program.cs conditional on '$(OutputType)' != 'Exe'?