test: uint256 - Garbage suffixes and zero padding

Summary:
This is a partial backport of core#30436
https://github.com/bitcoin/bitcoin/pull/30436/commits/2f5577dc2e7ba668798a89a2f6ef72795db6c285
Depends on D18870

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18871