CollectionService.findAllForSitemap(): add unit tests

The puzzle `1605-c1ea4a5f` from #1605 has to be resolved:

https://github.com/php-coder/mystamps/blob/adfb6ef571b9dfc9bd3adafb0b4156a0af7f5073/src/main/java/ru/mystamps/web/feature/collection/CollectionServiceImpl.java#L213-L213

Tech debt for: adfb6ef571b9dfc9bd3adafb0b4156a0af7f5073 (#1605)