[Tests] Change expected QAccessible::role to WebDocument.

That is the role that is returned from the WebView.

Change-Id: I9ab817e259c8fe69d756ffa576082066cf6b4ea8
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>