selftests: livepatch: add test cases of stack_order sysfs interface

Add selftest test cases to sysfs attribute 'stack_order'.

Suggested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Wardenjohn <zhangwarden@gmail.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20241024083530.58775-1-zhangwarden@gmail.com


[pmladek@suse.com: Substitute $SYSFS_KLP_DIR]
Signed-off-by: Petr Mladek <pmladek@suse.com>