[avalanche] Move gen_proof to avatools
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).
ninja all check-all
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Maniphest Tasks: T1634
Differential Revision: https://reviews.bitcoinabc.org/D10020