Add test for performing special operations on XSLTProcessor properties (#13959)

This covers some uncovered code paths in custom property handling in
php_xsl.c

ext/xsl/tests/special_operations_with_properties.phpt

@@ -0,0 +1,68 @@
+--TEST--
+Special operations with XSLTProcessor properties
+--EXTENSIONS--
+xsl
+dom
+--FILE--
+<?php
+
+$xml = new DOMDocument;
+$xml->loadXML('<?xml version="1.0"?><root><foo>hello</foo></root>');
+
+function test() {
+    echo "Called test\n";
+}
+
+$xsl = new DOMDocument;
+$xsl->loadXML(<<<XML
+<?xml version="1.0"?>
+<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:php="http://php.net/xsl" version="1.0">
+    <xsl:template match="/root">
+        <xsl:value-of select="php:function('test')"/>
+        <xsl:value-of select="//root/foo"/>
+    </xsl:template>
+</xsl:stylesheet>
+XML);
+
+echo "--- Unset cloneDocument ---\n";
+
+$xslt = new XSLTProcessor;
+$xslt->registerPHPFunctions();
+unset($xslt->cloneDocument);
+try {
+    $xslt->importStylesheet($xsl);
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+
+echo "--- Unset doXInclude ---\n";
+
+$xslt = new XSLTProcessor;
+$xslt->registerPHPFunctions();
+unset($xslt->doXInclude);
+$xslt->importStylesheet($xsl);
+try {
+    echo $xslt->transformToXml($xml);
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+
+echo "--- Make properties references ---\n";
+
+$xslt = new XSLTProcessor;
+$xslt->registerPHPFunctions();
+$clone =& $xslt->cloneDocument;
+$xinclude =& $xslt->doXInclude;
+$xslt->importStylesheet($xsl);
+echo $xslt->transformToXml($xml);
+
+?>
+--EXPECT--
+--- Unset cloneDocument ---
+Typed property XSLTProcessor::$cloneDocument must not be accessed before initialization
+--- Unset doXInclude ---
+Typed property XSLTProcessor::$doXInclude must not be accessed before initialization
+--- Make properties references ---
+Called test
+<?xml version="1.0"?>
+hello