Add more tests

[orhun]

Is your feature request related to a problem? Please describe.

The code coverage of this project is currently low. Needs more tests.

Describe the solution you'd like

Add unit/integration tests similar to https://github.com/orhun/systeroid/blob/81ec65099479a809e84e307d6cde3471870ddbe8/systeroid-tui/tests/integration_test.rs

Describe alternatives you've considered

None.

Additional context

See https://github.com/orhun/systeroid/tree/main/systeroid-tui/src