[avalanche] Actually finalize block in voting functional test

Summary:
The log clearly indicates the block was intended to be finalized here.
It gets to block acceptance but stops prematurely. This is not desirable
since it does not match expectations nor does it lend itself well to
expanding this test in the future.

Test Plan:

ninja
./test/functional/test_runner.py abc_p2p_avalanche_voting

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12875