It's not possible to fully test full page zoom on LayoutTests.
Using zoom: style does exercise some of the right code paths, but not all of them. When the user zooms, the page is unaware of any change: all the APIs that take and receive coordinates (e.g. event clientX, pageX, offsetX, and document.elementFromPoint()) continue to use "unzoomed" points.
However, if there is zoom style in the content, then those APIs take the zoom into account, and use zoomed points.
So what we need is a DRT API that allows a test to do what Safari does when a page gets zoomed.