Basic specs for Rubinius::FFI::Library#attach_function

I noticed `Rubinius::FFI::Library#attach_function` currently doesn't have any specs.  Here are some basic ones.  

This is a precursor to me trying to make a few small changes to `Rubinius::FFI` to make it more "compatible" with the `ffi` gem in hopes of making it easier for code to target both at the same time.  But obviously this pull request is not a change to the implementation - just a little preliminary spec coverage.