test_tmp_files_deleted_on_sigint intermittent failure

---- test_sort::test_tmp_files_deleted_on_sigint stdout ----
bin: "/target/x86_64-unknown-linux-musl/debug/coreutils"
mkdir: /tmp/.tmp5WpLJb/tmp_dir
run: /target/x86_64-unknown-linux-musl/debug/coreutils sort big_file_to_sort.txt --buffer-size=1 --temporary-directory=tmp_dir

thread 'test_sort::test_tmp_files_deleted_on_sigint' panicked at tests/uutests/src/lib/util.rs:424:35:
called `Option::unwrap()` on a `None` value
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/std/src/panicking.rs:697:5
   1: core::panicking::panic_fmt
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/core/src/panicking.rs:75:14
   2: core::panicking::panic
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/core/src/panicking.rs:145:5
   3: core::option::unwrap_failed
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/core/src/option.rs:2040:5
   4: core::option::Option<T>::unwrap
             at /rust/lib/rustlib/src/rust/library/core/src/option.rs:1003:21
   5: uutests::util::CmdResult::code
             at ./tests/uutests/src/lib/util.rs:424:9
   6: uutests::util::CmdResult::code_is
             at ./tests/uutests/src/lib/util.rs:436:21
   7: tests::test_sort::test_tmp_files_deleted_on_sigint
             at ./tests/by-util/test_sort.rs:1615:5
   8: tests::test_sort::test_tmp_files_deleted_on_sigint::{{closure}}
             at ./tests/by-util/test_sort.rs:1573:38
   9: core::ops::function::FnOnce::call_once
             at /rust/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
  10: core::ops::function::FnOnce::call_once
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

https://github.com/uutils/coreutils/actions/runs/25051929679/job/73382332993?pr=12056

Start with test_tmp_files_deleted_on_sigint in tests/by-util/test_sort.rs and the CmdResult::code unwrap at tests/uutests/src/lib/util.rs:424. Run the test repeatedly, using the linked GitHub Actions failure as context, and trace why the interrupted command has no code. Done means the intermittent failure is understood and the test passes reliably.