test_forward_sigint_to_child intermittent failure

---- test_timeout::test_forward_sigint_to_child 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 got_int; exit 42' INT; sleep 5

thread 'test_timeout::test_forward_sigint_to_child' panicked at tests/by-util/test_timeout.rs:287:10:
'' does not contain 'got_int'
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:759:9
   3: tests::test_timeout::test_forward_sigint_to_child
             at ./tests/by-util/test_timeout.rs:284:5
   4: tests::test_timeout::test_forward_sigint_to_child::{{closure}}
             at ./tests/by-util/test_timeout.rs:273:34
   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.

https://github.com/uutils/coreutils/actions/runs/24253366791/job/70819000100?pr=11330

Start with tests/by-util/test_timeout.rs around test_forward_sigint_to_child at lines 273-287, then inspect the linked CI failure and the timeout command shown in the log. Reproduce the test on the affected macOS environment and trace why stdout can omit "got_int". Done means the intermittent failure is understood and the test reliably verifies SIGINT forwarding.