This tests that the correspondingElement and
correspondingUseElement attributes of the
SVGElementInstance interface have expected values.
The test consists of a single rectangle that is a descendant
of a number of g elements and a couple of nested
use shadow trees. When the user clicks the rectangle,
a mousedown event is dispatched, which travels up
the nested shadow trees. If the correspondingElement and
correspondingUseElement attributes of the
SVGElementInstance object (which is the currentTarget
of the event) are correct, the rectangle becomes blue. Otherwise,
it becomes red.
