test: ensure tests pass when nokogiri uses system libxml2
test: ensure tests pass when nokogiri uses system libxml2

Specifically the patch that affects this behavior is

  nokogiri/patches/libxml2/0002-Update-entities-to-remove-handling-of-ssi.patch

which was introduced to avoid server-side-include vulnerabilities, see

  https://github.com/sparklemotion/nokogiri/commit/4852e43

rails/rails-html-sanitizer