SeriesInfoExtractorServiceImpl.extractSellerGroup(): add unit tests

[0pdd]

The puzzle 857-16517a02 from #857 has to be resolved:

mystamps/src/main/java/ru/mystamps/web/service/SeriesInfoExtractorServiceImpl.java

Line 318 in 95faa9a

// @todo #857 SeriesInfoExtractorServiceImpl.extractSellerGroup(): add unit tests

The puzzle was created by Slava Semushin on 15-Jul-18.