Fix ZBX-18478

See https://support.zabbix.com/browse/ZBX-18478

The patch fix ZBX-18478 problem.

After the "Test" or "Update" button is pressed and "LDAP Settings" form is refreshed, the cookie that holds the tab index is cleared. Therefore, the "Test" button is disappear from below  form. So, I fixed that the cookie value keep in a php variable.