autotests: Pass parent object as last argument to createXdgToplevelSurface()...

autotests: Pass parent object as last argument to createXdgToplevelSurface() and createXdgPopupSurface()

It's more common to see the parent object being the last argument in Qt
and this way you won't need to specify nullptr parent explicitly if the
xdg-popup or the xdg-toplevel surface doesn't need to be configured
implicitly, which makes tests slightly easier to read.