[avalanche] Improve coverage for the addavalanchenode RPC

Summary:
This diff adds a new functional test for this RPC. Next step is to improve the error message.

Ref T1635.

Test Plan:

./test/functional/test_runner.py abc_rpc_addavalanchenode

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Maniphest Tasks: T1635

Differential Revision: https://reviews.bitcoinabc.org/D9786