test/fuzz: add a seed corpus generation option to the test_runner
Summary:
This adds a startup option to test/fuzz/test_runner.py which allows to generate seed corpus to the passed seed_dir instead of using them.
Backport of core PR19659.
Depends on D8278.
Test Plan:
ninja bitcoin-fuzzers ./test/fuzz/test_runner.py -j 24 -g <new_corpus_path>
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D8279