[avalanche] Fix a wrong test in abc_rpc_avalancheproof

Summary: Make sure we are testing what we expect, i.e. node 1 (and not 0) turns the proof mature (which wait_for_proof doesn't do as it asserts as soon as the proof is found).

Test Plan:

./test/functional/test_runner.py abc_rpc_avalancheproof

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13028