Use zim-testing-suite in tests

[rgaudin]

Our reader tests currently uses test zim files from libkiwix's test folder. While those files are still present, I am not sure what the future is for those so we should switch to the more future-proof, stable and appropriate https://github.com/openzim/zim-testing-suite.

[mgautierfr]

more future-proof, stable and appropriate openzim/zim-testing-suite.

Future-proof and appropriate yes.
stable... not yet 😉

[rgaudin]

OK 😅 Well libkiwix isn't stable either as the example.zim was completely rewritten.

Don't intend to do that soon anyway.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be now be reviewed manually. Thank you for your contributions.