fix(carousel): replaced $timeout with $interval when it was wrong

I had a to tweak a bit two tests since $interval never throws an exception after flushing, hope I did it OK. 

This addresses #2454
Indeed, previously it wasnt doing the right thing because it should've been flushing the timeout but catching the exception.

What I meant is that there should be a flush here for the interval service so that it's actually a useful assertion.