implement function waitFor() for yui test

Hi YUI team,

Following this conversation https://github.com/yui/yui3/issues/1036#issuecomment-21749364 , i pull request to implement function waitFor() for the yui test module.

The aim of this function is to resume a waiting test, not based on a duration, but on a defined condition (for example when a div is visible, contains a defined text or when dom is ready). 

Not sure that the way I implemented it, is the best, but this working.

Your opinion on this subject ?

Julien