[avalanche] Add an handy way to build the quorum in the test

Summary: As per title. We will need to restart the node at some point in this test, so we'll need to be able to rebuild the quorum quickly.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8769