Hello,
You have a couple of options to achieve what you want.
The easiest one is to create a file, script.php (the name does not matter), inside the PrestaShop root. Its content should be:
<?php require dirname(__FILE__) . '/config/config.inc.php'; Db::getInstance()->execute('TRUNCATE ps_test1'); Db::getInstance()->execute('TRUNCATE ps_test2');
Then you can add it to the existing cronjobs (e.g.: * * * * * php <full_path_to_prestashop_root>/script.php). This should work as expected.
The other option would be to create a custom module that can:
1. Use the actionAdminControllerSetMedia hook, so it gets called each time you access an admin page
2. Create a Symfony Command that you can use in your cronjobs
If you want, I can provide some code for this option as well.