Add extension test

src/globals/ext-tests/iconv.php

@@ -0,0 +1,6 @@
+<?php
+
+declare(strict_types=1);
+
+assert(function_exists('iconv'));
+assert(iconv('UTF-8', 'CP437', 'foo') === 'foo');

src/globals/test-extensions.php

@@ -20,7 +20,7 @@
 // If you want to test your added extensions and libs, add below (comma separated, example `bcmath,openssl`).
 $extensions = match (PHP_OS_FAMILY) {
     'Linux', 'Darwin' => 'iconv',
-    'Windows' => 'amqp,apcu',
+    'Windows' => 'amqp,apcu,iconv',
 };
 
 // If you want to test lib-suggests feature with extension, add them below (comma separated, example `libwebp,libavif`).