#1682: fix unit tests

[Ifropc]

Part of #1682

Fixes unit tests, including restoring help tests

Why

Previously, we were using clap's get_matches_from that exits with code 0 (printing help message). This made our tests exit with code 0 as well (when calling --help).
As part of the change we handle DisplayError returning help message, and keeping existing behavior for all other clap errors.

Known limitations

Some win tests are failing

[leighmcculloch]

This tests are fixed in other PRs (see #1682 )

What needs to merge before this PR can be merged?

[Ifropc]

@leighmcculloch we can either merge all listed PRs at the same time or merge all of them here first

• #1682: fix unit tests #1734
• #1682: fix build tests #1736
• 1682 fix config/version tests #1737
• 1682 fix init test #1738

[elizabethengelman]

This is looking great! Thanks for digging into it - it was driving me crazy ;p

we can either merge all listed PRs at the same time or merge all of them here first

W/r/t when to merge this and the other PRs, i like the idea of merging those test fixing into this PR first.

[leighmcculloch]

Thanks for figuring out why these tests were preventing from running. Great find and learning for us all.

For the solution, I'd like us to reconsider the approach here, see the inline comment. Please lmk if there's a reason we can't do the error approach.

[leighmcculloch]

One ask, otherwise this looks good, very easy to follow.

[elizabethengelman]

This is looking good to me, besides @leighmcculloch 's comment about moving the HelpMessage switching to the top level. I wonder this PR can go in as-is, and that can be a follow up PR?

I'm also curious what the exit code should be when no --help flag is passed. I am currently seeing that scenario, as well as when a --help is passing both returning a 0 error code.