[avalanche] Cycle through a set of 8 nodes in multi_block_register test

Summary:
This is a more realistic test case. It also prepares the test suite to be able to handle quorums formed by too little nodes.

Depends on D2584

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2585