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.