Enable testServer usage with

Currently the `testServer` helper only works with declaratively register functions. We should find a way to make it work with the old background functions as well.

One idea is to have it accept a function as an argument instead of the registered name...

Start at the testServer helper entry point and trace how it resolves declaratively registered functions versus old background functions. Done means testServer can exercise a background function while preserving its existing behavior for declarative registrations.

索引モデルが issue の本文から書いたものです。