New SVG A11y tests for use/shadow DOM

The "use element testable statements" from the wiki (
https://www.w3.org/wiki/SVG_Accessibility/Testing/Test_Assertions_with_Tables)
will need to be updated.

We'll probably want to cover most combinations of the following conditions:

   - Does the <use> have an accessible name/description from a child
   title/desc?
   - Does the <use> have an accessible name/description from ARIA
   attributes?
   - Does the <use> element have explicit role="none"?
   - Does the <use> element have aria-hidden="true"?
   - Does the re-used element have an accessible name/description?
   - Does the re-used content include text?
   - Is the re-used element interactive (e.g., a link)?
   - Is the re-used element a symbol, a rendered shape/group, or a
   shape/group that is not rendered because it's inside a <defs>?

Considering all the interactions between those, we are looking at a few
dozen tests.

As for what's the correct behavior:

   - Under the new model, the <use> should be treated like a generic
   container for the re-used content.  Whether the container, or the content,
   or both, gets exposed in the accessibility tree depends on whether it has
   any alternative text, explicit role, or interactivity.
   - Role="none" makes the container transparent, but does not hide the
   content.
   - aria-hidden should hide the content unless it's interactive.
   - Whether the original element was rendered or not does not affect
   whether or how the cloned element is exposed.