chg: test: Refactor the forward ans6 response server

Rewrite the server in the declarative `isctest.asyncserver` style, one scoped handler per answered query instead of one branchy handler.

Includes a small `isctest.asyncserver` change — an `edns` attribute on `StaticResponseHandler` — because the two static handlers here need to advertise no EDNS declaratively rather than by overriding `get_responses()`. `forward/ans6` is its only user.

Merge branch 'stepan/forward-asyncserver-cleanup' into 'main'

See merge request isc-projects/bind9!12534