Remove Cli.init side effect from top level of library

Description / Motivation

This MR removes a side effect from the tezt-core lib which prevents it from being used in other binaries. The top level call to Cli.init would change the command line parsing of binaries that depend on it.

How to Test the MR Manually

dune exec test/unix/main.exe -- --help ## should show help
dune exec test/unix/main.exe -- ## should run test

Checklist

• Update CHANGES.md. No need to document changes to documentation and tests.
• Make sure all new values, types etc. are documented in .mli files and that the generated documentation looks ok.
• [-] Add tests in test/, if relevant.