Previous to this PR, `codex-rs/core/tests/sandbox.rs` contained
integration tests that were specific to Seatbelt. This PR moves those
tests to `codex-rs/core/src/seatbelt.rs` and designates
`codex-rs/core/tests/sandbox.rs` to be used as the home for
cross-platform (well, Mac and Linux...) sandbox tests.
To start, this migrates
`python_multiprocessing_lock_works_under_seatbelt()` from #1823 to the
new `sandbox.rs` because this is the type of thing that should work on
both Mac _and_ Linux, though I still need to do some work to clean up
the test so it works on both platforms.
a8c7f5391c