Axe-core may be too permissive about where/when to use the group role

[WilcoFiers]

The ARIA 1.3 editors draft has created some clarity on what is and isn't allowed to be done with groups. My new understanding of it is that you can put nest groups directly into some, but not all list-like roles. And depending on the list-type thing you might be able to nest groups.

So this stuff is valid:

tree > treeitem > group > treeitem
menu > group > group > menuitem
listbox > group > option
listbox > group > group > option

But none of the following are:

list > group > listitem
list > listitem > group > listitem
tree > group > group > treeitem
tablist > group > tab

This reverts a change we made in #2897. We made that change because APG had examples that put the group inside treeitem, but that seems to have been incorrect. I've opened up an issue with APG (w3c/aria-practices#2887). We should probably wait to see what comes out of that, and quite possibly not fix this until the W3C site has been updated.

Before we pull anything we also should test whether there are support problems with it.