test_terminate_child_on_receiving_terminate intermittent failure

---- test_timeout::test_terminate_child_on_receiving_terminate stdout ----
bin: "/Users/runner/work/coreutils/coreutils/target/x86_64-apple-darwin/debug/coreutils"
run: /Users/runner/work/coreutils/coreutils/target/x86_64-apple-darwin/debug/coreutils timeout 10 sh -c trap 'echo child received TERM' TERM; sleep 5

thread 'test_timeout::test_terminate_child_on_receiving_terminate' panicked at tests/by-util/test_timeout.rs:229:10:
'' does not contain 'child received TERM'
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: uutests::util::CmdResult::stdout_contains
             at ./tests/uutests/src/lib/util.rs:768:9
   3: tests::test_timeout::test_terminate_child_on_receiving_terminate
             at ./tests/by-util/test_timeout.rs:224:5
   4: tests::test_timeout::test_terminate_child_on_receiving_terminate::{{closure}}
             at ./tests/by-util/test_timeout.rs:213:49
   5: core::ops::function::FnOnce::call_once
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/core/src/ops/function.rs:250:5
   6: 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.

CICD / Build (macos-latest, x86_64-apple-darwin, feat_os_unix, true) (pull_request)

Start with tests/by-util/test_timeout.rs around test_terminate_child_on_receiving_terminate and the macOS CI failure linked in the issue. Reproduce the test on macOS, then inspect the timeout test's process and output expectations. Done means the intermittent failure is understood and the test passes reliably without losing the expected child output.