Core:CSS: Attach test nodes to documentElement, not body

Core:CSS: Attach test nodes to documentElement, not body

Attaching test divs to document.documentElement instead of document.body
used to cause issues in jQuery 1.x; jQuery Compat doesn't execute any tests
on document ready, though so it could be aligned with master.

This makes jQuery Compat support tests work correctly even if jQuery is
included & used in head before body even exists - making it similar to
the master behavior.

Fixes gh-2502