Make tests compatible with libxml2 2.9.12

This version of libxml introduced quite a few changes. Most of
them are differences in error reporting, while some also change
behavior, e.g

Make tests compatible with libxml2 2.9.12

This version of libxml introduced quite a few changes. Most of
them are differences in error reporting, while some also change
behavior, e.g. null bytes are no longer supported and xinclude
recursion is limited.

Closes GH-7030. Closes GH-7046.

Co-authored-by: Nikita Popov <nikic@php.net>

show more ...