MDL-70846 accessibility: update tree attributes to pass a11y check

- Move aria-* atrributes from <p> to <li>
- Move "role" attribute from <p> to <li>
- Update behat tests

Based on reference implementation from:
- https://www.w3.org/TR/wai-aria-practices-1.1/examples/treeview/treeview-2/treeview-2a.html
- https://www.w3.org/WAI/GL/wiki/Using_ARIA_trees

lib/amd/src/tree.js

@@ -269,7 +269,8 @@ define(['jquery'], function($) {
                 var moduleName = item.closest('[data-ajax-loader]').attr('data-ajax-loader');
                 var thisTree = this;
                 // Flag this node as loading.
-                item.addClass('loading');
+                const p = item.find('p');
+                p.addClass('loading');
                 // Require the ajax module (must be AMD) and try to load the items.
                 require([moduleName], function(loader) {
                     // All ajax module must implement a "load" method.
@@ -280,7 +281,7 @@ define(['jquery'], function($) {
                         thisTree.initialiseNodes(item);
                         thisTree.finishExpandingGroup(item);
                         // Make sure no child elements of the item we just loaded are tabbable.
-                        item.removeClass('loading');
+                        p.removeClass('loading');
                         promise.resolve();
                     });
                 });