Update avalanche integration test to use a quorum

Summary: Instead of one node, the test will now provide a quorum of 8 fake nodes to bitcoind so we can simulate more realitic conditions.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6548