Infra changes to unify the tests to be built against a common project.json.

  Infra changes to unify the  tests to be built against a common project.json.
   1. tests\src\Common\test_dependencies\project.json - To consume corefx and other nuget feeds

   2. tests\src\Common\empty\project.json - To build against the mscorlib facade,to use this just Add
            <ReferenceLocalMscorlib>true</ReferenceLocalMscorlib> to your project

Commit migrated from https://github.com/dotnet/coreclr/commit/71dd695faef9e6d310a2df612cb51462dfeefc98