[avalanche] Move gen_proof to avatools

Summary:
This makes it available for other tests as well. Note that the behavior is changed because it uses a single coinbase utxo by default (instead of 10 which was not necessary).

Ref T1634.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1634

Differential Revision: https://reviews.bitcoinabc.org/D10020