CategoryService.findAllForSitemap(): add unit test

The puzzle `1605-7e6638bf` from #1605 has to be resolved:

https://github.com/php-coder/mystamps/blob/2b4886618342017914c1b461068347a854210810/src/main/java/ru/mystamps/web/feature/category/CategoryServiceImpl.java#L120-L120

Tech debt for: 2b4886618342017914c1b461068347a854210810 (#1605)