tests: opcache_preload (#2257)

Adds a test to reproduce #2254 and verify `opcache_preload` works as
intended with the changes from
https://github.com/php/frankenphp/pull/2252.

testdata/preload.php

@@ -0,0 +1,8 @@
+<?php
+
+// verify ENV can be accessed during preload
+$_ENV['TEST'] = '123';
+function preloaded_function(): string
+{
+    return 'I am preloaded';
+}