Update test zim files.

This set illustration following the new spec with scale information.
(`M/illustration_48x48@1`)

It also use the last version (branch) of libzim/zim-tools to correctly
mark html file in small.zim as front article.

small.zim and invalid.* zim files are created using create_test_zimfiles.

wikibooks_be_all_nopic_2017-02.zim and
wikipedia_en_climate_change_nopic_2020-01.zim are generated using
zimrecreate tool, from "withns" zim files, zstd compression, no ftindex.

wikibooks_be_all_nopic_2017-02_splitted.zim* are generated using
zimsplit with a size of 51200.

Author: mgautierfr