Implement base integration tests

[mdrokz]

Changes

• move integration tests to main.rs since the crate is not configured as a library
• use TestBackend in tui crate to test the runner
• make render from the App struct public so we can render inside tests
• move test related dependencies to dev-dependencies section in cargo.toml

I basically use the render method to render the frames once and extract the text buffer to validate if it has the correct input. Later on we can write end-to-end tests to test the actual binary

Closes #13

[radlinskii]

Awesome! LGTM 🥳