Improve the test suite

[orhun]

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

binsider is not tested thoroughly right now. See the codecov report: https://app.codecov.io/gh/orhun/binsider

Describe the solution you'd like

Add unit and integration tests and assert the state of the TUI. See the following project's test suite for inspiration:

• https://github.com/orhun/systeroid
• https://github.com/kdheepak/taskwarrior-tui

Describe alternatives you've considered

None.

Additional context

There are some tests in place which should be skipped due to weird behavior on some platforms. For example, app::tests reads the debug binary but in CI it fails due to "resource temporarily unavailable". See https://github.com/orhun/binsider/actions/runs/10800912100/job/29959939240