Extending the Stories "deadline" from 24 hours to three days is significant because it's seen to appeal to Facebook's older audience who are less familiar with how Stories work.
The stories format, in general, is growing and could be the future of social sharing. In fact, stories are on track to overtake news feed posts as the most common way to share content. Making Stories last longer is a step in the right direction.
While Facebook didn't provide any information on the test, I'm sure they'll announce this within 2-3 months.