Don't assume any banscore in abc_p2p_avalanche_voting

Summary: That's no what we are testing and it is causing some flakiness on super slow machines (namely arm).

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_voting

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13230