Enhance integration test to assert final content of the ZIM

[benoit74]

https://github.com/openzim/warc2zim/issues/207 proved that warc2zim tests are not sufficient to ensure we produce a ZIM as expected under all conditions.

We could enhance zimit integration tests to also assert the list of entries in the ZIM, and more precisely the list of "special" entries for wombat and co, instead of looking only at how many WARC record have been processed.