1) Unless you have thousands of tests, it won't make much difference. (One text takes up so little space that "almost none" isn't far off.)
2) SMS Backup & Restore (free at the Play Store) has a "delete texts" button - it deletes them all.
3) Before you delete the texts, use it to make a backup (then store that file on a PC or somewhere and delete it from the phone), just in case you suddenly realize that one of those texts is the only copy you have of some very important information.