Add 'fs_portlet_add_remove_from_page' test case, covering 'fs_portlet::add_self_to_page', 'fs_portlet::remove_self_from_page', 'fs_contents_portlet::add_self_to_page', 'fs_contents_portlet::remove_self_from_page', 'fs_admin_portlet::add_self_to_page' and 'fs_admin_portlet::remove_self_from_page'