Fix test on alpine/musl (#15072)

When libxml uses musl iconv, GBK encoding is not supported as a target encoding

Author: arnaud-lb

ext/xmlwriter/tests/xmlwriter_toStream_encoding_gbk.phpt

@@ -0,0 +1,18 @@
+--TEST--
+XMLWriter::toStream() with encoding - test SHIFT_JIS
+--EXTENSIONS--
+xmlwriter
+--FILE--
+<?php
+
+$h = fopen("php://output", "w");
+
+$writer = XMLWriter::toStream($h);
+$writer->startDocument(encoding: "SHIFT_JIS");
+$writer->writeComment("\u{3041}\u{3041}\u{3041}");
+unset($writer);
+
+?>
+--EXPECT--
+<?xml version="1.0" encoding="SHIFT_JIS"?>
+<!--‚Ÿ‚Ÿ‚Ÿ-->