rpc, test: addpeeraddress test coverage, code simplify/constness

Summary:

- Add functional test coverage for rpc addpeeraddress
- Simplify addpeeraddress and improve code constness

Backport of core#22043.

Test Plan:

./test/functional/test_runner.py rpc_net

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12312