And actually, while you can't test against arena itself in trainer, you can use the custom AI button to simulate a level 80 arena opponent with a deck of your choice. Plenty of people have threads where they show off the decks they've submitted to platinum, so pick from those if you don't know the meta yourself.
edit: if it weren't for the fact that updates to trainer aren't even on the list of the top hundred things I'd like zanz to spend his coding time on, I'd say that it'd be neat to have the custom AI button updated to allow for other arena levels.