[avalanche] Fix dangling proof flakiness in contender test

Summary: The dangling proof cleanup may not occur until after the next block is mined which causes the test to fail the non-promotion check for the last old_quorum proof.

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/D18022