If you want to practice your answers you can do it in the actual interview page. Just record a video, but don't send it and it is viewable only to you. Interviewer can see your videos ONLY after you click Send-button. We also don't have any calculators where interviewers could see how many times you have recorded your answer. Unfortunately we don't have a possibility to save drafts so every time you record a new version the old one will be destroyed.
When you are practising make sure that there is enough light and that it comes from the front, your sound is clear and loud enough and also think about the posture and height of the camera. Our recommendation is to place the camera on eye-level.