test: use _ variable for unused loop counters (follow-up)

Summary: This is a follow-up to D10068, with unused loop counters found with the method described in D10344 (using vulture)

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10345