Add test for help function

[jemrobinson]

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:arrow_heading_up: Summary

:closed_umbrella: Related issues

Addresses point from #1853 review

:microscope: Tests

Tested locally

[jemrobinson]

In general, I don't like how cli.py is structured. It isn't very idiomatic typer.

I think it would be much clearer and easier to use if it followed the normal model of a typer.Typer object and typer.Typer.command decorators.

Are you happy to make that change on this PR?