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/[e-mail]
Reviewed-by: Oleg Nesterov <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

tools/testing/selftests/pidfd/pidfd_info_test.c

···
 	/* Opening a thread as a thread-group leader must fail. */
 	pidfd_thread = sys_pidfd_open(pid_thread, 0);
 	ASSERT_LT(pidfd_thread, 0);
+	ASSERT_EQ(errno, ENOENT);
 
 	/* Opening a thread as a PIDFD_THREAD must succeed. */
 	pidfd_thread = sys_pidfd_open(pid_thread, PIDFD_THREAD);