Fix remaining issues

[JordanMartinez]

Description of the change

• Updates tests to use Aff, so rather than printing something we then manually check is correct, an exception is thrown if the expected state isn't hit.
• Fix exitH's string value
• Add missing FFI for execSync'
• Drop ipc on *Sync functions (error at runtime if used in this way)
• Drop safeStdio, inline it
• Fix fromKillSignal FFI arg order

---

Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)