test: remove duplicate assertions in util_tests

Summary:
Co-authored-by: Kiminuo <kiminuo@protonmail.com>

This is a backport of core#21491
Depends on D11078 and D11081

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11082