Well of course Pinkie isn't going to ask a question that Trixie can't answer. That wouldn't be very nice at all!
And if Trixie were a bit smarter and a lot less cocky, she'd be pretty worried right now. Pinkie asking such a question suggests that Trixie has already failed the real test, and this is just meant to let her think she's passed.
Now I'm thinking that the crystal cage bit was a sort of backup test - even if Twi screwed up the initial meeting due to fatigue and stress, there's no way the real Twilight Sparkle could resist correcting such an elementary error.