In theory, it should. What we're trying to get at is that, when you type something, then press DownArrow, focus goes onto the first selected option. Accessibility focus, that is, the thing that gets set via aria-activedescendant. And that is definitely covered by tests. Likewise, if you hit left or right arrow, focus should go back to the edit box, e. g. aria-activedescendant should be removed. The tests cover prety thoroughly the cases we normally encounter when interacting with the keyboard. The new QuantumBar should behave in the same manner, e. g. emit the same accessibility events if possible.
So yes, this test is pretty thorough in what it covers. Right now, as in the latest Nightly with the fix for bug 1524561, it still behaves as before, e. g. up and down arrows simply speak the updated text in the edit box itself, not the actual result that has been selected.