Hidden images: add integration tests

[0pdd]

The puzzle 1356-b1ab6b12 from #1356 has to be resolved:

mystamps/src/main/webapp/WEB-INF/views/series/info.html

Line 171 in 01dca27

<!--/* @todo #1356 Hidden images: add integration tests */-->

The puzzle was created by Slava Semushin on 04-May-20.

[php-coder]

See also #1383