add some tests for values

cannot use __setitem__ to set value for an option
in append, len should not greater than master option

tiramisu/setting.py

@@ -223,7 +223,7 @@ class Settings(object):
         return Property(self, self._getproperties(opt, path), opt, path)
 
     def __setitem__(self, opt, value):
-        raise ValueError('you must only append/remove properties')
+        raise ValueError('you should only append/remove properties')
 
     def reset(self, opt=None, _path=None, all_properties=False):
         if all_properties and (_path or opt):