tests: simplify next_block() function in feature_block

Summary:
The solve parameter is unnecessary. Remove it and add
comments.

This is a backport of Core PR18366
Depends on D8846

Test Plan: ninja && test/functional/test_runner.py feature_block

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8847