fix: add missing property $context

See https://www.php.net/manual/en/class.streamwrapper.php#streamwrapper.props

In PHP 8.2, the following error ocurrs:
ErrorException: Creation of dynamic property CodeIgniter\Test\PhpStreamWrapper::$context is deprecated

Author: kenjis

system/Test/PhpStreamWrapper.php

@@ -20,6 +20,11 @@
  */
 final class PhpStreamWrapper
 {
+    /**
+     * @var resource|null
+     */
+    public $context;
+
     private static string $content = '';
     private int $position          = 0;