ExpandCatalogNumbersEditor: add unit tests

The puzzle `694-6574cec0` from #694 has to be resolved:

https://github.com/php-coder/mystamps/blob/712c62384be3decb641e8993a8223f01b30d2f29/src/main/java/ru/mystamps/web/controller/editor/ExpandCatalogNumbersEditor.java#L33-L33

The puzzle was created by Slava Semushin on 08-Jul-18.