Since I asked https://superuser.com/a/1282119 I reinstalled my computer from scratch, therefore I cannot test the answer(s) provided.
How should I handle them? never accept any, since it's not tested?
As someone who's asked a question on the site, you have a few options as to what you can do with each answer:
Of those, "Downvote it and accept the answer" seems really strange to me, but the rest are at least, in theory, a reasonable action you could take.
Adopting a personal policy like "I only accept answers if I can actually go back and test them properly" indicates a level of rigor and personal responsibility that's nice, but not required. In fact, you're perfectly entitled to upvote and accept an answer that you know is wrong, and the site will not punish you for that.
The general feeling on the site is that posts (questions and answers) eventually reach a vote score befitting the actual merit/value of the content, sort of like how companies eventually settle into equilibrium of the true value of that company on the stock market.
So in the short term, a good answer will be unfairly scored low (0 by default), and a bad answer will be unfairly scored high (it should be negative, and deleted if egregiously bad or harmful to the site).
In the long term, the community's collective wisdom will eventually determine the "correct" score of the answer, as well as your question. If no votes come its way, that just means nobody really cares about the subject matter, in which case there's no point worrying about it.
Folks accessing the site trying to gain knowledge from your question and the answers to it will tend to trust the vote totals over the green checkmark. The accepted answer checkmark is indicative of the opinion (for whatever that's worth) of the original question asker, as to which answer is best. I, and most rational SE users, prefer to trust the collective wisdom of the community over a single person's opinion.
In terms of community etiquette, it's generally a good idea not to intentionally mislead people. If you want to avoid misleading others into thinking that bad answers are good, or that unverified answers are verified, you might want to hold off on accepting the answer if you can't verify that it's correct. But that's "common sense"; the use of common sense, community etiquette, etc. is neither expected nor required to use the site, and abandoning it won't lead to you getting punished.
In short, it doesn't really matter. Accept it or don't as you see fit. You could accept it even if you can't test it, if you feel that the answer was written well enough and plausible enough that you'd like to reward the answerer with some reputation. Or you could not accept it to maintain some kind of personal code of rigor about your accepted answers.
Or you could accept it because Tim Post lost his keys.