Process test: wait until the detached child has left the group

`a_detached_pipe_cannot_hang_completion` started a `setsid` child and
exited after a fixed 0.1 s. On a slow runner the child had not called
setsid() yet, so the runner's group kill took it too, the pipe closed,
and the output was not marked truncated. The shell now exits once the
child has written its pid, which it does after setsid.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Fn7NbipAkYG7cYZxHk8xRu

Author: claude

native/core/tests/process_stress.rs

@@ -117,7 +117,11 @@ async fn a_detached_pipe_cannot_hang_completion() {
     let root = tempfile::tempdir().unwrap();
     let result = run(
         ProcessSpec::shell(
-            "setsid sh -c 'echo $$ > escaped.pid; sleep 8' & sleep .1; printf done",
+            // Exit only once the child has left the shell's process group
+            // (it writes its pid after setsid); a fixed pause could end the
+            // shell first on a slow runner, and the group kill took it too.
+            "setsid sh -c 'echo $$ > escaped.pid; sleep 8' & \
+             while [ ! -s escaped.pid ]; do sleep .02; done; printf done",
             root.path().into(),
             Duration::from_secs(3),
         ),