What do you call a question you know the answer to, but you want an answer?

It's a "leading" question or an "interrogating" question. It is designed to prompt a specific confirmatory response.

However, if those seem too general or insufficient, you might consider "verifying", "substantiating, or "confirmatory" as adequate answers. The truth is already known and verification/confirmation is sought.

6.You respond, "That was a(n) verifying question"

6.You respond, "That was a(n) confirmatory question"

https://en.wikipedia.org/wiki/Leading_question

https://www.thefreedictionary.com/interrogating

https://www.thefreedictionary.com/verifying

https://www.thefreedictionary.com/confirmatory

https://www.thefreedictionary.com/substantiating

I wonder if what you're really looking for is the answer that you know, or the name of a question that you ask to check on someone. You could say that it was really a test.

In the event of catching a lying partner, you would say that you are performing a test since you know the truth and are testing to see if they will speak the truth or lie instead. Because you know the truth, and the other person knows the truth, but they might not know you know the truth, we're testing their truthfulness.

So:

A: Did you go to sleep early last night?
B: Yeah, I was tired.
A: That was a test. I know you didn't.
(argument ensues)

I think I would call it a type of loaded question. The reason I say "type of" is because it doesn't exactly fit the more common understanding of "loaded question", which is described by Wikipedia as:

Aside from being an informal fallacy depending on usage, such questions may be used as a rhetorical tool: the question attempts to limit direct replies to be those that serve the questioner's agenda.[2] The traditional example is the question "Have you stopped beating your wife?" Whether the respondent answers yes or no, he will admit to having a wife and having beaten her at some time in the past. Thus, these facts are presupposed by the question, and in this case an entrapment, because it narrows the respondent to a single answer, and the fallacy of many questions has been committed.[2] The fallacy relies upon context for its effect: the fact that a question presupposes something does not in itself make the question fallacious. Only when some of these presuppositions are not necessarily agreed to by the person who is asked the question does the argument containing them become fallacious.[2] Hence the same question may be loaded in one context, but not in the other. For example, the previous question would not be loaded if it were asked during a trial in which the defendant had already admitted to beating his wife.[2]

However, the OP's question is "loaded", in that the person being questioned is not fully aware that the questioner has a wider agenda in asking it. It sounds innocent enough, but is loaded with implication, and it is likely to explode!