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

locale に依存する SKIPIF を含む ext/xml/tests/xml007.phpt から始め、PHP 8.2 以降で失敗を再現します。strtoupper に関連する ext/xml の大文字・小文字変換の動作を追跡し、テストが実行され、非 ASCII 文字に対して locale に依存しない大文字処理が行われることを確認します。