[avalanche] Add test case of vote status for manually added stake winner

Summary: Improve test coverage slightly.

Test Plan:

ninja check-avalanche-stakecontendercache_tests

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16876