Can you delete all text messages at once on 4gLTE?

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.