Add tentative WPT for flat ARIA tree accessibility subtree

[HaTheo]

This PR adds a tentative test to establish and confirm expected browser conformance for flat ARIA tree structures as discussed in w3c/aria#2091.

Specifically, it introduces treeitems.tentative.html to verify that a flat DOM architecture utilizing role="tree" and role="treeitem" with explicit aria-level attributes correctly computes into the appropriate hierarchical accessibility subtree. This test defines a flat DOM tree of Fruits and Vegetables, with children Fruit (Apple, Pear, oranges) and Vegetables (Carrots, Broccoli, Spinach) respectively. It assigns aria-level="1" to top-level categories and aria-level="2" to their respective children, along with aria-setsize and aria-posinset properties consistent with the tree.

And utilizes AriaUtils.verifyAccessibilitySubtree to assert that the browser reflects the intended parent-child relationships and accessibility tree structure, despite the non-nested DOM layout tree items.

[spectranaut]

It is so awesome to see the new test functionality used for this!!

But I agree with your question on the spec change, I'm not sure if this means we should reorder the accessibility tree, it doesn't actually say in the spec. Let's wait for clarity on that.