FilesystemImagePersistenceStrategy.replacePreview(): add unit tests

[0pdd]

The puzzle 1303-59dd45e7 from #1303 has to be resolved:

mystamps/src/main/java/ru/mystamps/web/feature/image/FilesystemImagePersistenceStrategy.java

Line 131 in 4c2104a

// @todo #1303 FilesystemImagePersistenceStrategy.replacePreview(): add unit tests

The puzzle was created by Slava Semushin on 28-Apr-20.