selftests/pidfd: adapt to recent changes

selftests/pidfd: adapt to recent changes

Adapt to changes in commit 9133607de37a ("exit: fix the usage of
delay_group_leader->exit_code in do_notify_parent() and pidfs_exit()").

Even if the thread-group leader exited early and succesfully it's exit
status will only be reported once the whole thread-group has exited and
it will share the exit code of the thread-group. So if the thread-group
was SIGKILLed the thread-group leader will also be reported as having
been SIGKILLed.

Link: https://lore.kernel.org/r/20250403-work-pidfd-fixes-v1-1-a123b6ed6716@kernel.org
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>