Update framework references for product, add local refs for tests

Update the framework references for product assemblies to the correct
(updated) folder structure of the framework *.nupkg files (with a +
separating platforms rather than a -), and also add references to
nuget delivered .NET Core framework contracts to test assemblies.
Fix SIMD test and disable XML tests on issue corefx/208

https://github.com/dotnet/corefx/issues/208 tracks running tests
against the implementation assemblies for .NET Core instead of the
desktop assemblies.  Disable some tests from running because they
compile against a new overload in the System.Runtime contract
(System.String System.String.Format(System.IFormatProvider,
System.String, System.Object) that does not exist in the desktop
implementation assembly (which only has a params overload).