Add egui testing library (#5166)

- closes #3491 
- closes #3926

This adds a testing library to egui based on
[kittest](https://github.com/rerun-io/kittest). Kittest is a new
[AccessKit](https://github.com/AccessKit/accesskit/)-based testing
library. The api is inspired by the js
[testing-library](https://testing-library.com/) where the idea is also
to query the dom based on accessibility attributes.
We made kittest with egui in mind but it should work with any rust gui
framework with AccessKit support.

It currently has support for:
- running the egui app, frame by frame
- building the AccessKit tree
- ergonomic queries via kittest
  - via e.g. get_by_name, get_by_role
- simulating events based on the accesskit node id
- creating arbitrary events based on Harness::input_mut
- rendering screenshots via wgpu
- snapshot tests with these screenshots

A simple test looks like this: 
```rust
fn main() {
    let mut checked = false;
    let app = |ctx: &Context| {
        CentralPanel::default().show(ctx, |ui| {
            ui.checkbox(&mut checked, "Check me!");
        });
    };

    let mut harness = Harness::builder().with_size(egui::Vec2::new(200.0, 100.0)).build(app);
    
    let checkbox = harness.get_by_name("Check me!");
    assert_eq!(checkbox.toggled(), Some(Toggled::False));
    checkbox.click();
    
    harness.run();

    let checkbox = harness.get_by_name("Check me!");
    assert_eq!(checkbox.toggled(), Some(Toggled::True));

    // You can even render the ui and do image snapshot tests
    #[cfg(all(feature = "wgpu", feature = "snapshot"))]
    egui_kittest::image_snapshot(&egui_kittest::wgpu::TestRenderer::new().render(&harness), "readme_example");
}
```

~Since getting wgpu to run in ci is a hassle, I'm taking another shot at
creating a software renderer for egui (ideally without a huge dependency
like skia)~ (this didn't work as well as I hoped and it turns out in CI
you can just run tests on a mac runner which comes with a real GPU)
 
Here is a example of a failed snapshot test in ci, it will say which
snapshot failed and upload an artifact with the before / after and diff
images:

https://github.com/emilk/egui/actions/runs/11183049487/job/31090724606?pr=5166

crates/egui_demo_lib/Cargo.toml

@@ -55,8 +55,13 @@ serde = { workspace = true, optional = true }
 
 
 [dev-dependencies]
-criterion.workspace = true
+# when running tests we always want to use the `chrono` feature
+egui_demo_lib = { workspace = true, features = ["chrono"] }
 
+criterion.workspace = true
+egui_kittest = { workspace = true, features = ["wgpu", "snapshot"] }
+wgpu = { workspace = true, features = ["metal"] }
+egui = { workspace = true, features = ["default_fonts"] }
 
 [[bench]]
 name = "benchmark"

crates/egui_demo_lib/src/demo/demo_app_windows.rs

@@ -377,3 +377,52 @@ fn file_menu_button(ui: &mut Ui) {
         }
     });
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::demo::demo_app_windows::Demos;
+    use egui::Vec2;
+    use egui_kittest::kittest::Queryable;
+    use egui_kittest::Harness;
+
+    #[test]
+    fn demos_should_match_snapshot() {
+        let demos = Demos::default();
+
+        let mut errors = Vec::new();
+
+        for mut demo in demos.demos {
+            // Remove the emoji from the demo name
+            let name = demo
+                .name()
+                .split_once(' ')
+                .map_or(demo.name(), |(_, name)| name);
+
+            // Widget Gallery needs to be customized (to set a specific date) and has its own test
+            if name == "Widget Gallery" {
+                continue;
+            }
+
+            let mut harness = Harness::new(|ctx| {
+                demo.show(ctx, &mut true);
+            });
+
+            let window = harness.node().children().next().unwrap();
+            // TODO(lucasmerlin): Windows should probably have a label?
+            //let window = harness.get_by_name(name);
+
+            let size = window.raw_bounds().expect("window bounds").size();
+            harness.set_size(Vec2::new(size.width as f32, size.height as f32));
+
+            // Run the app for some more frames...
+            harness.run();
+
+            let result = harness.try_wgpu_snapshot(&format!("demos/{name}"));
+            if let Err(err) = result {
+                errors.push(err);
+            }
+        }
+
+        assert!(errors.is_empty(), "Errors: {errors:#?}");
+    }
+}

crates/egui_demo_lib/src/demo/text_edit.rs

@@ -112,3 +112,37 @@ impl crate::View for TextEditDemo {
         });
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use egui::{accesskit, CentralPanel};
+    use egui_kittest::kittest::{Key, Queryable};
+    use egui_kittest::Harness;
+
+    #[test]
+    pub fn should_type() {
+        let mut text = "Hello, world!".to_owned();
+        let mut harness = Harness::new(move |ctx| {
+            CentralPanel::default().show(ctx, |ui| {
+                ui.text_edit_singleline(&mut text);
+            });
+        });
+
+        harness.run();
+
+        let text_edit = harness.get_by_role(accesskit::Role::TextInput);
+        assert_eq!(text_edit.value().as_deref(), Some("Hello, world!"));
+
+        text_edit.key_combination(&[Key::Command, Key::A]);
+        text_edit.type_text("Hi ");
+
+        harness.run();
+        harness
+            .get_by_role(accesskit::Role::TextInput)
+            .type_text("there!");
+
+        harness.run();
+        let text_edit = harness.get_by_role(accesskit::Role::TextInput);
+        assert_eq!(text_edit.value().as_deref(), Some("Hi there!"));
+    }
+}

crates/egui_demo_lib/src/demo/widget_gallery.rs

@@ -285,3 +285,31 @@ fn doc_link_label_with_crate<'a>(
         })
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::View;
+    use egui::{CentralPanel, Context, Vec2};
+    use egui_kittest::Harness;
+
+    #[test]
+    pub fn should_match_screenshot() {
+        let mut demo = WidgetGallery {
+            // If we don't set a fixed date, the snapshot test will fail.
+            date: Some(chrono::NaiveDate::from_ymd_opt(2024, 1, 1).unwrap()),
+            ..Default::default()
+        };
+        let app = |ctx: &Context| {
+            CentralPanel::default().show(ctx, |ui| {
+                demo.ui(ui);
+            });
+        };
+        let harness = Harness::builder()
+            .with_size(Vec2::new(380.0, 550.0))
+            .with_dpi(2.0)
+            .build(app);
+
+        harness.wgpu_snapshot("widget_gallery");
+    }
+}

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