Add separate page on how to test UI

[kdheepak]

Currently an example for how to write tests is only shown in the tutorial. It would be nice to have it as a separate page to make it easier for search engines to find it. It'll also be easier to read for users who are interested in just that one aspect of Ratatui

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn render() {
        let app = App::default();
        let mut buf = Buffer::empty(Rect::new(0, 0, 50, 4));

        app.render(buf.area, &mut buf);

        let mut expected = Buffer::with_lines(vec![
            "┏━━━━━━━━━━━━━ Counter App Tutorial ━━━━━━━━━━━━━┓",
            "┃                    Value: 0                    ┃",
            "┃                                                ┃",
            "┗━ Decrement <Left> Increment <Right> Quit <Q> ━━┛",
        ]);
        let title_style = Style::new().bold();
        let counter_style = Style::new().yellow();
        let key_style = Style::new().blue().bold();
        expected.set_style(Rect::new(14, 0, 22, 1), title_style);
        expected.set_style(Rect::new(28, 1, 1, 1), counter_style);
        expected.set_style(Rect::new(13, 3, 6, 1), key_style);
        expected.set_style(Rect::new(30, 3, 7, 1), key_style);
        expected.set_style(Rect::new(43, 3, 4, 1), key_style);

        // note ratatui also has an assert_buffer_eq! macro that can be used to
        // compare buffers and display the differences in a more readable way
        assert_eq!(buf, expected);
    }
}

[kdheepak]

This should probably go into how-to/develop-apps: https://github.com/ratatui-org/ratatui-website/tree/main/src/content/docs/how-to/develop-apps

[EdJoPaTo]

Should probably wait until ratatui/ratatui#1007 is settled.

[kdheepak]

Turns out this is a duplicate of this issue: #358