[avalanche] Move VoteRecord tests to their own file

Summary:
VoteRecord needs better test coverage on its own. Shoveling more tests into processor_tests is bad idea,
so this patch separates VoteRecord tests into their own file.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11499