Add a variance test

It wasn't immediatly obvious to me while starring at the code that the case where a parent type is not loadable is handled.

Just needed to understand that get_class_from_type() only returns NULL when it is not a class-type

Author: Girgias

Zend/tests/type_declarations/intersection_types/variance/invalid6.phpt

@@ -0,0 +1,20 @@
+--TEST--
+Replacing not-loadable parent intersection type with loadable child intersection type
+--FILE--
+<?php
+
+// Let Y and Z be loadable.
+interface Y {}
+interface Z {}
+
+class Test {
+    function method(): X&Y {}
+}
+class Test2 extends Test {
+    function method(): Y&Z {}
+}
+
+?>
+===DONE===
+--EXPECTF--
+Fatal error: Could not check compatibility between Test2::method(): Y&Z and Test::method(): X&Y, because class X is not available in %s on line %d