Add tests for `language-pack` command

[swissspidy]

Issue Overview

Especially needed because of this bug where $projects is immediately overridden, so using --all doesn't really work:

https://github.com/wearerequired/traduttore/blob/a9faec1a8e3b3ddbcb5222d12c2c3bbf17f069fc/inc/CLI/LanguagePackCommand.php#L228-L247

To Reproduce Steps to reproduce the behavior:

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2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior A clear and concise description of what you expected to happen.

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Versions

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• PHP:
• WP-CLI:
• GlotPress:
• WordPress:

Additional context