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

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