Add test for Stringable with optional interfaces

Author: tontonsb

Zend/tests/optional_interfaces/stringable.phpt

@@ -0,0 +1,20 @@
+--TEST--
+Adding stringable is not broken with Optional interfaces
+--FILE--
+<?php
+
+trait Str
+{
+    public function __toString() {}
+}
+
+class TestClass implements ?NonexistantInterface
+{
+    use Str;
+}
+
+echo implode(',', class_implements('TestClass'));
+
+?>
+--EXPECT--
+Stringable