test: add ParseUInt8() unit and fuzz test coverage

Summary:
Also reuse test string with embedded null char in other tests.

This is a backport of core#21438

Test Plan: ninja check bitcoin-fuzzers

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11081