[avalanche] Increase quorum size in the test.

Summary: 8 is the bare minimum for things to work, and consideing this isn't what we are testing for, it is better to test using a larger quorum.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6659