fix(test): make test/inner.rs more robust

Summary

• fix(test): make test/inner.rs more robust

  • Use bare exec in inner.rs instead of fork + exec.
  • Use parent death signal on linux (man 2 PR_SET_PDEATHSIG).
  • Fix usage of waitpid() (man 2 waitpid).

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• Close #...
• Cherry-pick to: none / 24.6
• Docs follow-up: not necessary / new issue
• Changelog is updated