if your domain is
/var/www/vhosts/test.com/httpdocs
and your sub domain is
/var/www/vhosts/test.com/subdomains/testing/httpdocs
then your relative path from /var/www/vhosts/test.com/subdomains/testing/httpdocs to /var/www/vhosts/test.com/httpdocs would be ../../../httpdocs/