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 that this erases the whole exam (not just one section) and cannot be undone.
The scores will be erased, however, your skills will not return to the level they were prior to taking the Practice exam.
For more information, check out this article: What if I have to leave before the time is up?