ext/xml case folding broken for non-ASCII characters

Description

See ext/xml/tests/xml007.phpt

The code will never run in 8.2 or up because of changes to how strtoupper works. This means the test will be skipped due to the SKIPIF settings which has a locale-dependent strtoupper check.
Removing the SKIPIF shows that the test is indeed broken, even on systems where it worked on 8.1 it won't work on 8.2.

Other than that, this never worked properly because it depends on the locale.

From https://www.w3.org/TR/WD-xml-970807.xml

case-folding: a process applied to a sequence of characters, in which those identified as non-uppercase (in scripts which have case distinctions) are replaced by their uppercase equivalents, as specified in The Unicode Standard, Version 2.0, section 4.1. Note that Unicode recommends folding to lowercase; for compatibility reasons, XML processors must fold to uppercase. Case-folding, as described here, neither requires nor forbids the normalization of Unicode character sequences into canonical form (e.g. as described in The Unicode Standard, section 5.9).

So it's not supposed to depend on the locale.

PHP Version

8.1 / 8.2 depending on your point of view

Operating System

No response

Start with ext/xml/tests/xml007.phpt, including its locale-dependent SKIPIF, and reproduce the failure on PHP 8.2 or newer. Trace the ext/xml case-folding behavior related to strtoupper and verify that the test runs and produces locale-independent uppercase handling for non-ASCII characters.