System.Reflection.Metadata.Tests: reference System.Threading contract assembly

This allows the test project to be built with Mono's xbuild.

Otherwise I get an error: `CSC: error CS0518: The predefined type 'System.Threading.Interlocked' is not defined or imported` from the Mono compiler.

I suspect this is because the [test project](https://github.com/dotnet/corefx/blob/532860a8912d7ede2106e2d8d40e202c1be35f8a/src/System.Reflection.Metadata/tests/System.Reflection.Metadata.Tests.csproj#L117-L124) builds against the contract assemblies from the NuGet packages, while the referenced [library project](https://github.com/dotnet/corefx/blob/532860a8912d7ede2106e2d8d40e202c1be35f8a/src/System.Reflection.Metadata/src/System.Reflection.Metadata.csproj#L13-L14) builds against the PCL profile and there is a mismatch somewhere. Would love to get some feedback if this makes sense :)