Add an additional test for concat_function for non-interned strings

This tests for the case where result == op1, op1 != op2 and op1_string
and op2_string are non-interned strings that are equal.

Author: nielsdos

Zend/tests/class_toString_concat_non_interned_with_itself.phpt

@@ -0,0 +1,21 @@
+--TEST--
+Test concatenating a class instance that has __toString with itself that uses a non-interned string
+--FILE--
+<?php
+$global_non_interned_string = str_repeat("a", 3);
+
+class Test {
+	public function __toString() {
+		global $global_non_interned_string;
+		return $global_non_interned_string;
+	}
+}
+
+$test1 = new Test;
+$test2 = new Test;
+$test1 .= $test2;
+
+echo $test1 . "\n";
+?>
+--EXPECT--
+aaaaaa