[avalanche] Reduce scope of block tips being checked in avalanche voting test

Summary: This patch makes more explicit which tip is being tested for avalanche acceptance. Keeping track of the outer-scoped hash_tip variables leads to headaches when making changes to this file.

Test Plan:

ninja && ./test/functional/test_runner.py abc_p2p_avalanche_voting

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13149