This feature is quite self-explanatory; there are many people who really want to be able to reset their (unit) tests, especially the longer ones, after making an error. 

I'm creating this feature request here because it has already demonstrated to be very highly sought-after in this topic:
https://khanacademy.zendesk.com/hc/en-us/community/posts/115001114611-Reset-Unit-Tests?page=5#comments

For some reason it is completely being ignored by Khan Academy up to a point where conspiracy theories are now starting to form. I honestly just don't get why there is no response from Khan Academy that explains for what reason this hasn't been implemented yet.

Anyways, so much has been said about this already in the previously mentioned topic. I'm going to quote a few things from there that I think are relevant. I'd really recommend reading the topic though, as there is a lot more info on this feature there and I can't quote everything ;)

pstriebig5383 - "Everyone I've ever known unanimously agrees that the school system's grades are, at best, an absolutely awful way of representing someone. If I could check my notes or look up how to do something, fix my answer, and not have my teacher put an 80% on my report card just because I missed one character, I'd have learned so much more than by either putting in fake answers until I get 100% or just taking the hit on my grade and getting an 80%. In practice, learners will either make their mistake and move on, accepting that it will hurt the grade that will follow them through the rest of their lives, or they will put in fake answers to reset and get a 100%. Neither of these benefit the learner, and adding a reset button would make no difference other than save hours apon hours of life."

Ronekk Ronekk - "If khan academy is trying to gamify learning, you're pretty successful because I'm about to ragequit."

Paul Sappington - "Level up on the original skill, lose points on another skill. Repeat round and round for three hours before finally reaching 100% or taking a baseball bat to the computer. A reset option would drastically reduce the needless waste."

Luke Inkson - "I agree 100% this would be a fantastic feature. I'm not a student, I'm simply refreshing my mathematics skills and plan to make my way through all of the available Khan Academy courses eventually - purely out of my own personal interest. I am somewhat OCD when it comes to testing, and would like to see 100% in each unit. However, I don't have an unlimited amount of time (juggling work and life) and when I make a silly mistake (or an accidental touch on my tablet) I know I have to waste time answering a bunch of questions with no hope of getting 100% on that try. It feels like a punishment, when learning should be fun!"

Lucy - "I can no longer suffer this 30-question quiz I've been trying to complete tonight... for one careless mistake I have to do it all over again. An early mistake translates to dozens of bogus answers which have a negative impact on my mastery challenges."

Aside from this, there are also a lot of thoughts on how it could function but since development doesn't response it's quite hard to know what, if any, problems they have with this.