As you could probably guess from the title, me and a lot more people (like Guillaume Juge, Edge, Tuffskulz, Knowledge Seeker, SupreJagger, Andrew Boÿe, Horizon, Sally Buss, Amanda Ashley, Aadyot Bhardwaj, Mostyn Jones, hanz64AH and Mike Shelton to name a few users I found who posted about this) are frustrated with the fact that if you or a student of yours gets a question wrong at any time throughout a Test/Quiz you have to finish the entire thing before restarting, and this leads to careless errors which in turn results in literally hours more work. One of the common requests to fix this is to add a restart/second try button for Quizzes, unit tests, etc.
I'm sure many more people will request this feature in the future and I will look forward to seeing it soon!
P.S. If I find a post about this by someone not named above I will try to add it!
P.P.S. there is a well upvoted post with like 110+ votes so here it is: https://khanacademy.zendesk.com/hc/en-us/community/posts/115001114611-Reset-Unit-Tests