tests: Add key_io fuzzing harness. Fuzz additional functions in existing fuzzing harnesses.
Summary:
Add key_io fuzzing harness. Fuzz additional functions in the hex fuzzing harness. Fuzz additional functions in the integer fuzzing harness. Fuzz additional functions in the script fuzzing harness. Fuzz additional functions in the transaction fuzzing harness.
Backport of core PR17926.
Test Plan:
ninja bitcoin-fuzzers ./test/fuzz/test_runner.py <path_to_corpus>
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D8267