Fix issues related to libxml2 2.12.0 (#12802)

* Avoid passing NULL to xmlSwitchToEncoding

This otherwise switches to UTF-8 on libxml2 2.12.0

* Split tests for different err

Fix issues related to libxml2 2.12.0 (#12802)

* 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

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Test fixes for libxml2 2.12.0

Fix #81351: xml_parse may fail, but has no error code

The fix for bug #73151[1] cured the symptoms, but not the root cause,
namely xmlParse() must not be called recursively. Since that

Fix #81351: xml_parse may fail, but has no error code

The fix for bug #73151[1] cured the symptoms, but not the root cause,
namely xmlParse() must not be called recursively. Since that bugfix
also messed up the error handling, we basically revert it (but also
simplify the return), and then prevent calling the parser recursively.

[1] <https://github.com/php/php-src/pull/2166/commits/f2a8a8c068995a5d780882c556cedd53bce3827d>

Co-authored-by: Nikita Popov <nikita.ppv@gmail.com>

Closes GH-7363.

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