Hey Euro... no, deadly serious. And I wasn't aware that it was about showing off ones taste in expensive clothing but in fact about making a 'good' impression! So one's fiscal position 'don't enter into it', you can pick up a smart pair of trousers/a skirt for a lot less than a decent pair of jeans as I'm sure you're aware?
The truth is, no-one takes actual note of who is wearing what on these aptitude test days and probably wouldn't know an individual applicant from Adam (or Eve) But that's not the point....
Want to make a good impression: DON'T WEAR JEANS! Want to look like you don't really give a toss and this is just one of many unimportant jobs that you're applying for; then be my guest.