Allow copy-local ReferenceFromRuntime

Allow copy-local ReferenceFromRuntime

Tests may want to copy a ReferenceFromRuntime to the test output directory.

To do this, a test can set Private=true (same as you would for a Reference).

By default we don't do this, because anything in the runtime folder will be
part of the shared framework that the test runs on.

In the case of Microsoft.XmlSerializer.Generator it's reference is an
application and the project expects this to be next to the test assembly.

Commit migrated from https://github.com/dotnet/corefx/commit/a938b3b95063b442981078b9d11ff66296daf51f