Bug 1038178 - [Vertical Homescreen] Use scrollbar from root scrollframe

https://bugzilla.mozilla.org/show_bug.cgi?id=1038178
Is it ok that this test no longer checks that this caused the page to scroll up? (and just that it scrolled at all) I guess it does what the test says it does and it's unlikely that we'll break this and make it scroll downwards, but still.