CollectionService.findSeriesInstances(): add unit tests

[0pdd]

The puzzle 1123-296bb340 from #1123 has to be resolved:

mystamps/src/main/java/ru/mystamps/web/feature/collection/CollectionServiceImpl.java

Line 135 in c3e11aa

// @todo #1123 CollectionService.findSeriesInstances(): add unit tests

The puzzle was created by Slava Semushin on 28-Dec-19.