[tests] use uint256_hex instead of repeating the string formatting pattern

Summary:
Follow-up to D11868

This change makes the code more readable by abstracting the int-to-hex string formatting.

Test Plan:
ninja check-functional-extended

grep -r "064x" test/functional
grep -r "0{64}x" test/functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13130