[avalanche] Verify that polling has started on an item before verifying that it has stopped

Summary:
If the node registers the proof too slowly, it's possible for the stale check to complete
all 5 retries before polling has even started. This is rarity, but may be one of the root
causes of flakiness that we're seeing on CI.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_proof_voting

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11800