[test] Speed up fuzzing by ~200x when using afl-fuzz

Summary:

Enable the `afl-clang-fast++` features deferred forkserver
(`__AFL_INIT`) and persistent mode (`__AFL_LOOP(1000)`).

Backport of core PR10415:
https://github.com/bitcoin/bitcoin/pull/10415/files

Test Plan:
Follow the fuzzing.md to run the fuzzer.
Check that the performance are improved by using afl-clang-fast.
My numbers:

• Before: ~60/s
• After: ~7300/s

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4578