[ticket/10972] Add destroy method to mock cache.

I actually needed the version that destroys tables, therefore
I ended up writing a mock null cache. This code is currently unused
but will probably be handy at some point.

PHPBB3-10972

tests/mock/cache.php

@@ -34,6 +34,16 @@ class phpbb_mock_cache
 		$this->data[$var_name] = $var;
 	}
 
+	public function destroy($var_name, $table = '')
+	{
+		if ($table)
+		{
+			throw new Exception('Destroying tables is not implemented yet');
+		}
+
+		unset($this->data[$var_name]);
+	}
+
 	/**
 	* Obtain active bots
 	*/