There are various of reasons to print test messages from your iPhone. However, there is no direct way to print iPhone test messages. If you have to print these messages for future use, you may try the methods below.

Method 1: Take a Screenshot
Open the text messages that you want, and press "Sleep/Wake" button and "volume key" at the same time to take a screenshot of sms messages, then send them to yourself by email to print.

Method 2: Print iPhone Text Messages Using Free iTunes Backup Extractor
This software is an ultimate free tool to explore, view, extract and retrieve data from iTunes backup of your iPhone, iPad & iPod Touch.
Note: Before using this software, you are required to backup your iPhone with iTunes.
Download and install this software - select a backup file - select "Messages&Attachments" to start scanning - After scanning, click "Recover" to