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
Shouldn't it be `$interval.flush(1000)`? 

Looking at it more closely I don't get why here we use $apply here to set scope.interval value and it's directly modified on the last test as `scope.interval = 2000;`