Add unit and UI tests on a recent sample ZIM file

[mossroy]

Our current unit and UI tests use an old ZIM file : wikipedia_en_ray_charles_2015-06.zim

Since then, the format and contents have evolved : at least zstd compression format, no-namespace ZIM flavor, and webp. These are implemented in kiwix-js, but not covered by our current automatic tests.

While keeping the current tests on the old ZIM file (for non-regression), we should add some on a more recent ZIM file.
Ideally, it would be on the same "Ray Charles" content, so that we could use the same scenario. I've opened openzim/zim-testing-suite#5 to have a suitable ZIM file.

[Jaifroid]

We already had #718 for this, but I think this issue is more generic, so I'll close that one.

[Jaifroid]

I think this issue has superseded #456 because it makes sense to continue testing on the old split ZIM for non-regression (as you say, @mossroy) while adding a modern ZIM for no-namespace, zstandard and webp. This gives better coverage than simply updating Ray Charles,

[mossroy]

It would be important to keep the step that checks some images dimensions in the UI scenario, on this newer ZIM file.
Because, when run on browsers that do not support webp (IE11, Firefox 45), it would check that the webp polyfill works properly (instead of implementing #674)

[Jaifroid]

Closed in favour of #1042.