I'm going to do user tests on a Chorded/Chording Keyboard. I would like to know, if there are any guidelines / tips for testing user-input (example sentences to input)?
I'm going to assume your aim is to determine whether a given physical/digital chorded keyboard device has usability issues when used by users who are familiar with using a chorded keyboard (e.g. court stenographers). Your situation might be, for example, a need to evaluate various replacement devices due to obsolescence or usability issues with the current deployed devices.
Research the current situation to determine what factors are more important than others, and typical usage scenarios. Things like speed of entry vs accuracy, short bursty dictation vs sustained endurance-testing transcription, level of ambient distractions, and so on.
Talk to existing users - find out what issues are being reported with any current equipment, find out how they became proficient, how they determine their proficiency level. Compare and contrast the user's comments with comments from managers, stake holders, project sponsors etc - there may be hidden differences in values to be resolved, and there may be usability issues being hidden by workarounds. Find out what class of errors are occurring frequently, and which are causing the most problems.
Decide on the primary purpose of testing. Consider whether you are testing for purposes of discovering as-yet-unknown problems, or testing relative quantitative qualities (speed, accuracy, etc). (aka Formative vs Summative usability testing)
Shape and design your usability tests based on your research. These factors will determine what minimum ratings for accuracy and speed are to be tested, and what relative weightings to assign to any analysis of results. You also want to be sure you'll test relevant scenarios, and not wasting time/effort testing irrelevant scenarios.
Watch out for the Experience Law of Learning if doing repeated tests with the same subjects/participants.
Look to training materials for example texts to test with.