There have been many requests to reset scores and Khan Academy staff have responded to these requests by adding a feature to reset a full test. You can find the link to reset the scores at the bottom of a test page.
You will get a pop-up before your scores are erased. Note carefully that this erases the whole exam (not just one section) and cannot be undone.
For more information, check out this article: What if I have to leave before the time is up?