[avalanche] set a minimum amount for a proof

Summary:
This bumps the dust threshold per UTXO in a proof to 1 coin and fixes tests accordingly.

PROOF_DUST_THRESOLD is moved from proof.cpp to proof.h so it is available for tests.

Depends on D9441

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta, deadalnix

Reviewed By: #bitcoin_abc, majcosta, deadalnix

Subscribers: deadalnix, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9345