That's funny- I was looking at tennis skirts today and thinking hmmmmm.....Jodhpurs, too....
I used to test in a black skirt/dress, tights and a white mock turtleneck zip up sweater, with white gloves. Still have the gloves!
Soooo what you normally wear sounds fine to me! Especially your tennis skirt doesn't have shorts under it (or if they won't be seen) and everything's in subdued colors. Just maybe switch out for a nice fitted sweater? Or, depending on where the dress you have is too tight, could you get away with it underneath a sweater?
I would be nervous about the tie around skirts, just because they seem so fiddly and I always fear they'll come undone. But if that's what you have, it looks nice, and you get the nod from your coach, then why not?
But what is your rink culture? I'd ask around there, too.....there might be some different customs where you are now. If it comes to it, could you borrow a dress from one of your fellow skaters?
I think what's important is to look nice for the judges, to be comfortable and in subdued colors so all of you can focus on the important thing: your skating. I wouldn't wear to much, if any bling for a test.
Good luck on Saturday!!!