svgTitleText naming method is too permissive

**Expected**: In an `svg` element, only a direct child `title` element can provide an accessible name

**Actual**: Axe-core allows any descendant title element to provide an accessible name for `svg` elements. I think the same might be true for `fieldset > legend` and `figure > figcaption`.

We should do some testing to make sure this causes problems and that AT aren't more permissive than the accName spec says they should be.