Improve test coverage for unexpected HTML entity escaping in libxml 2.7.5 and later

Set up xss_foliate in the test context "these tests should pass for libxml 2.7.5 and later". This doesn't fix the tests, but it does make them properly fail.

See my comment on loofah at https://github.com/flavorjones/loofah/issues/20#issuecomment-1751538 for more details…