Test that DOM className helpers work correctly with SVGs.
Fix SVG creation in tests.

SVG elements must be created via document.createElementNS, with the correct
namespace ( http://www.w3.org/2000/svg ); if created with document.createElement,
they are not SVG nodes but Unknown Element nodes (i.e. their prototype is
HTMLUnknownElementPrototype instead of SVGSVGElementPrototype).