`zetacore` : replace `require` testing package with `assert`

[kingpinXD]

[lumtis]

I think we can group it with #1697 since ZetaClient doesn't have many tests for now. Also, please, better to use zetacore and zetaclient labels instead of precising it in the title for triaging.

[kingpinXD]

Closing the issue , we have moved to using required on zetacore