The _TEST_EMAIL_DEST constant is used for testing dispatched emails, by replacing any and all email recipient addresses with that defined by the constant. For example, if you insert the following code in to your mes_options.php (see The mes_options.php file) file:
define('_TEST_EMAIL_DEST', 'administrator@mydomain.test');
Then it will be the "administrator" email account that will receive all emails sent out by SPIP (subscription emails, notification emails, etc).
To ensure that no one at all receives any email, you can also insert the following code:
define('_TEST_EMAIL_DEST', '');
If you wish, you may also specify several addresses by separating them with commas.