[tests]: CLI testing

[vhyrro]

This will be a massive undertaking, but once the project matures a bit more we'll have to invest some time in testing the CLI for various edge cases, not just the library.

[vhyrro]

This is becoming more important now than ever, so we should start considering this as "on our radar' :)

[mrcjkb]

It would be nice to set up a mock server (like rocks-binaries) with mock packages to install, so we can keep the tests reproducible.