test: fix flaky test_env_arg_ignore_signal_valid_signals timing issue

The test was using a fixed 100ms delay after sending a signal, which was
not sufficient on slower systems or under CI load. This caused the test
to fail intermittently when the process hadn't exited yet.

Changed to use a retry loop with 50 iterations of 10ms delays (total 500ms
max wait time), which gives the process more time to handle the signal and
exit while still being responsive.

This fixes the flaky test failure on macOS CI.

Author: naoNao89

tests/by-util/test_env.rs

@@ -48,7 +48,14 @@ impl Target {
             .spawn()
             .expect("failed to send signal")
             .wait();
-        self.child.delay(100);
+        // Give the process time to handle the signal and exit
+        // Use a longer delay with retries to handle slower systems
+        for _ in 0..50 {
+            self.child.delay(10);
+            if !self.child.is_alive() {
+                break;
+            }
+        }
     }
     fn is_alive(&mut self) -> bool {
         self.child.is_alive()