CRM_Core_OptionValue::addOptionValue crashes if it tries to create a new option group

If you try to create an option value with CRM_Core_OptionValue::addOptionValue() where the option group does not yet exist, the code attempts to create the new option group for you. However, Civi crashes when it gets to an invocation of CRM_Utils_Weight::getDefaultWeight (currently at line 204), because require_once() is never called in that code path.

I enclose a patch, and a PHPUnit test that exposes the issue on the unpatched code.