I had forgotten that we had a `TestDouble.extend_onto` API.  I originally added this to support rspec-fire awhile back for `class_double` situations, but it was before I realized you can subclass `Module`, which is a better approach (and the approach we use now).

Given that, we may want to do away with `extend_onto` as an API...but that's a separate issue from this PR.  Just musing :).