Deferred tree

Purpose: This is similar to the deferred animation test case. The animation controller is not created for every web page, but only for those pages that contain SMIL animatable content. But, if some SVG content containing animation is added after the page is loaded, the animation should still run.

Operation: There is a plain XHTML document, but after 1s an SVG document is added. This document contains a moving circle.

Expected results: After 1s, an SVG document fragment appears containing a circle that moves downward.

NOTE: This test case is currently broken. The event used for initialising animation will not fire when a tree is added entirely via script. This will be addressed at a later stage when we consider how to implement externalResourcesRequired (see #277955).