test_env_arg_ignore_signal_valid_signals intermittent failure

---- test_env::test_env_arg_ignore_signal_valid_signals stdout ----
bin: "/Users/runner/work/coreutils/coreutils/target/debug/coreutils"
run: /Users/runner/work/coreutils/coreutils/target/debug/coreutils env --ignore-signal=int sleep 1000
bin: "/Users/runner/work/coreutils/coreutils/target/debug/coreutils"
run: /Users/runner/work/coreutils/coreutils/target/debug/coreutils env --ignore-signal=usr2 sleep 1000
bin: "/Users/runner/work/coreutils/coreutils/target/debug/coreutils"
run: /Users/runner/work/coreutils/coreutils/target/debug/coreutils env --ignore-signal=int,usr2 sleep 1000

thread 'test_env::test_env_arg_ignore_signal_valid_signals' (101074) panicked at tests/by-util/test_env.rs:897:9:
assertion failed: !target.is_alive()
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/e408947bfd200af42db322daf0fadfe7e26d3bd1/library/std/src/panicking.rs:689:5
   1: core::panicking::panic_fmt
             at /rustc/e408947bfd200af42db322daf0fadfe7e26d3bd1/library/core/src/panicking.rs:80:14
   2: core::panicking::panic
             at /rustc/e408947bfd200af42db322daf0fadfe7e26d3bd1/library/core/src/panicking.rs:150:5
   3: tests::test_env::test_env_arg_ignore_signal_valid_signals
             at ./tests/by-util/test_env.rs:897:9
   4: tests::test_env::test_env_arg_ignore_signal_valid_signals::{{closure}}
             at ./tests/by-util/test_env.rs:883:46
   5: core::ops::function::FnOnce::call_once
             at /rustc/e408947bfd200af42db322daf0fadfe7e26d3bd1/library/core/src/ops/function.rs:250:5
   6: core::ops::function::FnOnce::call_once
             at /rustc/e408947bfd200af42db322daf0fadfe7e26d3bd1/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

L10n (Localization) / L10n/Build and Test (macos-latest, feat_os_unix) (pull_request)

Start in tests/by-util/test_env.rs around test_env_arg_ignore_signal_valid_signals, especially lines 883-897, and run the targeted test on macOS. Investigate why the sleep process remains alive after the env --ignore-signal cases. Done means the intermittent failure is understood and the test passes reliably.