[avalanche] Remove getPeerId

Summary:
This accessor does more that its name implies, as it attempts to create a peer if it doesn't exists which makes the API confusing. This was intended to be a test only method, but it sneaked outside of its original scope.

This diff removes getPeerId() and use registerProof() instead when this is the expected behavior. The original behavior is now enforced as test-only by moving it to the friend test class.

Ref T1854.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: majcosta

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10440