TimedDownloaderService: add unit tests

The puzzle `665-32370c4c` from #665 has to be resolved:

https://github.com/php-coder/mystamps/blob/829173f0df88aa1dc2c974618757b22d68ea7465/src/main/java/ru/mystamps/web/service/TimedDownloaderService.java#L34-L34

The puzzle was created by Slava Semushin on 02-Jan-18.