[web/test] stop calling loadxhtml with a form.callback
It was deprecated in 3.9 (changeset 7b9553a9db65) and removed in 3.18.
Related to #5533303.
# -*- coding: utf-8 -*-
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s schema"""