[avalanche] Properly validate proof in peermanager.cpp
Summary:
The proof needs to match some existing UTXO.
The tests spoof the UTXO set to make it work.
Depends on D8533
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D8534