test/fuzz: add a seed corpus generation option to the test_runner
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 [[ https://github.com/bitcoin/bitcoin/pull/19659 | 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

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