behat_base::spin could be improved to allow micro sleep

While working on course management behat tests I had a need to use spin.
I found there was one limitation, it uses sleep so the min wait time is 1s.
I needed to watch for a DOM change that would last for just 1s and as such came up with this modification.

Really it will allow us to tweak for performance more easily at a later date and at the same time also adds the ability to switch a closure for an array that maps to a class method to be called instead.