A test pilot was a pilot who tested new prototypes. (DIS: "Light and Shadows")
In 2143, Test pilots Jonathan Archer and A.G. Robinson test piloted the NX-Alpha and NX-Beta and achieved breaking the Warp 2 barrier. (ENT: "First Flight")
After graduating from Starfleet Academy, one of Christopher Pike's first assignments in Starfleet was test piloting new classes of shuttlecraft. (DIS: "Light and Shadows")
Michael Burnham was selected to test pilot the landing pods. (DIS: "Brother")
In 2374, When the lifeform who was impersonating the individual Steth was brought aboard USS Voyager, he said he was a test pilot. (VOY: "Vis à Vis")