If you listen to Prof Akşit, you will see that they already have 3 TF6000 prototypes. They are conducting different tests on them. The engine has been engineered to provide 6000lbf thrust. They already have attained 3800lbf thrust from the engine. By testing it and gradually increasing its output they will reach the programmed thrust levels.
Today they may have 3800lbf thrust. But they check to see the wear and tear on the engine parts and correct if any parts are not performing to specs and upgrade it accordingly and test it again. By using three prototype engines they are trying to cut down the time needed for the tests.