Improve test coverage of `onefetch`

We're lacking both integration and unit tests to make sure we won't run into regressions after new development/refactoring.

Modules that should be covered by unit tests:

• cli.rs

UI

• ui/mod.rs
• text_colors.rs
• printer.rs

Image backends

• image_backends/mod.rs
• iterm.rs
• kitty.rs
• sixel.rs

Info

• info/mod.rs
• info_field.rs
• title.rs

Repo info fields

• repo/mod.rs
• author.rs
• commit.rs
• contributors.rs
• created.rs
• head.rs
• last_change.rs
• license.rs
• loc.rs
• pending.rs
• project.rs
• size.rs
• url.rs
• version.rs

Dependencies (package manager)

• deps/mod.rs
• package_manager.rs

Language

• langs/mod.rs
• language.rs

Modules that should be covered by integration tests:

• info/mod.rs
• printer.rs (pointers --> https://github.com/o2sh/onefetch/pull/699#issuecomment-1179406932)
• git.rs (pointers --> https://github.com/o2sh/onefetch/pull/705#issuecomment-1193484501)

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