[test] Vote unknown for the elements we don't care in abc_p2p_avalanche_contender_voting

Summary:
This will avoid finalizing/rejecting inventory items inadvertenly while the test expects to finalize/invalidate only some.

This is a step towards fixing the test flakiness.

Depends on D18037.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_contender_voting

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D18038