Test for "Allow treeitems to omit groups by specifiying aria-level, aria-posinset, and aria-setsize"

[spectranaut]

See: w3c/aria#2091

If we land the computed accessibility properties (see list: w3c/aria#2800 (comment)) then we will be able to test with level!! :)

But, this test could also be made with an aamtest. Seems more appropriate for computed accessibility properties, however.