Update arg test; add config tests

[ClementTsang]

Description

A description of the change and what it does. If relevant, please provide screenshots of what results from the change:

With 1.4.3 we can avoid the weird test-binary thing.

Furthermore, finally add config tests, and do a refactor of current error handling.

Issue

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Closes: #

Type of change

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• Other (something else - please specify if relevant):

Test methodology

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Please tick which platforms this change was tested on:

• Windows
• macOS
• Linux

Checklist

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• Change has been tested to work
• Areas your change affects have been linted using rustfmt
• Code has been self-reviewed
• Code has been tested and no new breakage is introduced unless intended
• Documentation has been added/updated if needed
• No merge conflicts arise from the change

Other information

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