Take a guess.

You've seen this request many times over the last half of a decade.

You've seen how many people want this feature.

You've already shot down this request, and I don't care.

Adding a restart option to save users the compounding frustration of having to complete entire Unit Tests just to get the option to do them again over a single mistake unrelated to the learning material is -- in the most literal sense -- the least you could possibly do -- the absolute laziest, fastest, and most simple option you have to mitigate this issue. I've seen your responses and reasons for not adding this feature, and they blatantly ignore the heaps of valid reasons many users have given over the past FIVE YEARS -- the cookie-cutter over-use of "we want to promote a growth mindset" in particular drives me nuts. I'm not even going to start with the explanations I have for why this is by far the most basic way of fixing the issue, and why it should be implemented rather than your complete refusal to acknowledge the issue -- because you have them all already. If you want my reason, you can try not ignoring your userbase and actually read their posts. If you want to give me the same cookie-cutter response that you give everyone else, then you can just not respond to me at all -- it'd mean the same thing.

Sincerely -
          - A user who is deeply frustrated by your complete dismissal of the topic