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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Implement DOMNode::isEqualNode()

Since we still support obsoleted nodes in our implementation, this uses
the old spec to match the old nodes; and this uses the new spec for
nodes sti

Implement DOMNode::isEqualNode()

Since we still support obsoleted nodes in our implementation, this uses
the old spec to match the old nodes; and this uses the new spec for
nodes still defined in the living spec.
When unclear, the behaviour was cross-verified with Firefox.

References:
https://dom.spec.whatwg.org/#dom-node-isequalnode (for everything still in the living spec)
https://www.w3.org/TR/2004/REC-DOM-Level-3-Core-20040407/DOM3-Core.html#core-Node3-isEqualNode (for old nodes removed from the living spec)

Closes GH-11690.

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