test: add test coverage for CNetAddr ipv6 scoped addresses

Summary:
This is a backport of core#19951 [1/2]
https://github.com/bitcoin/bitcoin/pull/19951/commits/5cb5fd3005435f3a7ca0c3401951d1db8db4fb88

Test Plan: ninja check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10425