Updating tests.js with background-image property.

Great work by Lea Verou with CSS3test, tests.js was missing background-image property with it's 
parameters, as (url(), none, inherit). It consists of background property, which sets all background-property in just one declaration.
Background-image property is a valid spec. as listed by w3.org ( http://www.w3.org/TR/css3-background/#the-background-image )
& w3schools.com ( http://www.w3schools.com/cssref/pr_background-image.asp ), 
review it..