* Avoid passing NULL to xmlSwitchToEncoding
This otherwise switches to UTF-8 on libxml2 2.12.0
* Split tests for different error reporting behaviour in libxml2 2.12.0
* Avoid deprecation warnings for libxml2 2.12.0
We can't fully get rid of the parser globals as there are still APIs
that implicitly use them.
* Temporarily disable part of test for libxml 2.12.0 regression
See https://gitlab.gnome.org/GNOME/libxml2/-/issues/634
* Review fixes
* [ci skip] Update test description
