[avalanche] Add some tests to check that duplicate votes are not counted

Summary:
Every change we make to vote accounting risks breaking vote deduping.
This patch adds some test coverage to reduce that risk since I plan on doing
just that.

Depends on D11499

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11500