Attempt to fix gh11498.phpt on MSAN

The issue might be that due to slow instrumentation the process might end before
we get to add it to the processes list. If the SIGCHLD handler executes before
adding the process to the list it will never be removed again.

Author: iluuu1994

ext/pcntl/tests/gh11498.phpt

@@ -19,7 +19,8 @@ pcntl_signal(SIGCHLD, function($sig, $info) use (&$processes) {
 }, false);
 
 for ($i = 0; $i <= 5; $i++) {
-    $process = proc_open('echo $$ > /dev/null', [], $pipes);
+    // Sleeping ensures we get to add the process to the list before the signal is invoked.
+    $process = proc_open('sleep 1', [], $pipes);
     $pid = proc_get_status($process)['pid'];
     $processes[$pid] = $process;
 }