Add tests for CSS multi-columns

Making a new Pull Request for this as it's been awhile and rebasing from a year ago doesn't seem like fun (especially with all the **v3** jazz). 

I've added tests that modify the initial CSS Multi-Columns implementation, very similar to the way video is handled (as per @paulirish's suggestion). Ran tests and everything looked green. 

Let me know if you have any feedback @slexaxton

Here is the old PR discussion for reference: https://github.com/Modernizr/Modernizr/pull/737