[fuzzing] Use subprocess.Popen call directly, remove use of thread pool. #20751

This removes the threadpools from test/fuzz/test_runner.py.

Each thread just spawns a subprocess, there's no reason to have both a thread and subprocess.

This needs a bit more code, so absent any performance speedup, I'd prefer the previous version. My assumption was that the fuzzing happens in the subprocess, so any python GIL "deadlocks" in the thread pool won't have any performance effect on the subprocess itself. I might be wrong, though.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #20759 (doc: [test] Remove outdated comment in fuzz runner by MarcoFalke)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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🐙 This pull request conflicts with the target branch and needs rebase.

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