Properly check the node polls for contenders upon restart with no prior block being mined

Summary:
The abc_p2p_avalanche_contender_voting.py test incorrectly assumes that no block is mined after the node restarted. But blocks are mined to generate the coinbase transactions required for staking when building the quorum.

Instead we can prepare the proofs before the restart and assign them to the peers in order to build the quorum with no block being mined.

Loosely depends on D18027 (another fix for this test).

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_contender_voting

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Subscribers: roqqit

Differential Revision: https://reviews.bitcoinabc.org/D18028