[ticket/10972] Add mock null cache.

The mock cache has instrumentation methods and therefore is non-trivial
to implement. For those times when we don't care that the cache caches,
null cache is a simpler implementation.

PHPBB3-10972

tests/mock/null_cache.php

@@ -0,0 +1,42 @@
+<?php
+/**
+*
+* @package testing
+* @copyright (c) 2012 phpBB Group
+* @license http://opensource.org/licenses/gpl-2.0.php GNU General Public License v2
+*
+*/
+
+class phpbb_mock_null_cache
+{
+	public function __construct()
+	{
+	}
+
+	public function get($var_name)
+	{
+		return false;
+	}
+
+	public function put($var_name, $var, $ttl = 0)
+	{
+	}
+
+	public function destroy($var_name, $table = '')
+	{
+	}
+
+	public function obtain_bots()
+	{
+		return array();
+	}
+
+	public function obtain_word_list()
+	{
+		return array();
+	}
+
+	public function set_bots($bots)
+	{
+	}
+}