[avalanche] Fix the avalanche_proof_inventory test

Summary:
An orhpan is created using an hardcoded amount. This amount is BCHA and
causes the proof to be below dust threshold.

Test Plan:

./test/functional/test_runner.py abc_p2p_proof_inventory

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9670