fuzz: Run in parallel

Summary:
Backport of core PR18688.

Depends on D8273 and D8276.

Test Plan:

ninja bitcoin-fuzzers
./test/fuzz/test_runner --par=24 <path_to_corpus>

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8277