Consider adding a shipping package baseline test

As part of the postmortem for a 5.0.1 issue where we needed to mark Identity.Specficiation.Tests as shipping. It seems worth adding a test with baselines to track the total set of packages we ship, similar to how we track the list of assemblies in the shared fx here: https://github.com/dotnet/aspnetcore/blob/master/src/Framework/test/TestData.cs.

The analogy would be something that just enumerates all the nupkgs in the shipping artifacts and fails if there's a mismatch from the list so we can track/be explicitly intentional about changes in the packages we ship.