Add an additional test for concat_function

This tests the path op1_string_from_func == true, result == op1,
op1 == op2. This wasn't tested before.

Author: nielsdos

Zend/tests/class_toString_concat_with_itself.phpt

@@ -0,0 +1,16 @@
+--TEST--
+Test concatenating a class instance that has __toString with itself
+--FILE--
+<?php
+class Tmp {
+	public function __toString() {
+		return "abc";
+	}
+}
+
+$tmp = new Tmp;
+$tmp .= $tmp;
+echo $tmp . "\n";
+?>
+--EXPECT--
+abcabc