test: Add fuzzing harnesses for various classes/functions in util/

Summary:
Backport of core PR18736.

2 commits have been skipped because irrelevant:

• tests: Add fuzzing coverage for StringForFeeReason(...)
• tests: Sort fuzzing harnesses

Test Plan:

ninja bitcoin-fuzzers
./test/fuzz/test_runner.py <path_to_corpus>

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9015