[avalanche] Fix staking reward flakiness after node restart in contenders test

Summary: Staking rewards may not be ready and trying to force them to be ready via setmocktime after restarting the node causes the prior quorum's proofs to be removed for dangling too long. Instead, we can check if staking rewards is ready and verify the respective poll response. Either PENDING or ACCEPTED are reasonable for our purposes here.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_contender_voting

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17997