[avalanche] Set voterecord_tests node ID using a fixture

Summary:
This puts shared init code for each test in a common fixture and clarifies some naming.

Depends on D11502

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11503