Check that we get somewhat sane PIDs when spawning with pidfds

library/std/src/sys/pal/unix/linux/pidfd/tests.rs

@@ -21,6 +21,7 @@ fn test_command_pidfd() {
         let flags = super::cvt(unsafe { libc::fcntl(pidfd.as_raw_fd(), libc::F_GETFD) }).unwrap();
         assert!(flags & libc::FD_CLOEXEC != 0);
     }
+    assert!(child.id() > 0 && child.id() < -1i32 as u32);
     let status = child.wait().expect("error waiting on pidfd");
     assert_eq!(status.code(), Some(1));
 
@@ -47,6 +48,8 @@ fn test_command_pidfd() {
     let mut child =
         unsafe { Command::new("false").pre_exec(|| Ok(())) }.create_pidfd(true).spawn().unwrap();
 
+    assert!(child.id() > 0 && child.id() < -1i32 as u32);
+
     if pidfd_open_available {
         assert!(child.pidfd().is_ok())
     }