I wouldn't tell her. You don't have to lie - just use it as an opportunity to create intrigue
if she asks you so. If she doesn't, then don't mention it. If you already said that to her and she's a very inexperienced girl, then you might get away with it, but generally girls prefer men who are more sexually experienced than they are.
Here's what you could say in case she asks you:
"I don't kiss and tell" (This one is classic)
"That's a secret... we'd have to go somewhere more secluded for me to tell you [sly smile]"
"Damn, we barely met and you're already thinking of yourself as 'the one'? I don't take girls as girlfriends before taking a 'test-drive', you know... [said playfully]"
First example: I'd say is a very neutral response to her shit test. You frame as if you did kiss other girls in the past but wouldn't tell, which is a desirable trait for a lover and girls get this, and at the same time you don't answer her so you still have the upper hand and now she may insist you tell her, which is something you want to capitalize on.
On the second example you out-frame her test by inviting her to do something with you - hopefully something that moves things towards your bedroom. If she insist that you say something, treat that as an window for you to escalate things forward. Get up and say "come with me here for a sec and I'll tell you."
On the third one you disqualify yourself as a boyfriend and make herself justify/get along with your joke, framing you as the prize.