selftest/pidfd: add test for thread-group leader pidfd open for thread

Verify that we report ENOENT when userspace tries to create a
thread-group leader pidfd for a thread pidfd that isn't a thread-group
leader.

Link: https://lore.kernel.org/r/20250403-work-pidfd-fixes-v1-4-a123b6ed6716@kernel.org


Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>