Add Harness::new_eframe and TestRenderer trait (#5539)

Co-authored-by: Andreas Reich <r_andreas2@web.de>

crates/egui_demo_app/tests/snapshots/clock.png

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+size 327802

crates/egui_demo_app/tests/snapshots/custom3d.png

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+size 93118

crates/egui_demo_app/tests/snapshots/easymarkeditor.png

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+size 182833

crates/egui_demo_app/tests/snapshots/imageviewer.png

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+version https://git-lfs.github.com/spec/v1
+oid sha256:e6cc6ff64eb73ddac89ecdacd07c2176f3ab952c0db4593fccf6d11f155ec392
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crates/egui_demo_app/tests/test_demo_app.rs

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+use egui::accesskit::Role;
+use egui::Vec2;
+use egui_demo_app::{Anchor, WrapApp};
+use egui_kittest::kittest::Queryable;
+
+#[test]
+fn test_demo_app() {
+    let mut harness = egui_kittest::Harness::builder()
+        .with_size(Vec2::new(900.0, 600.0))
+        .wgpu()
+        .build_eframe(|cc| WrapApp::new(cc));
+
+    let app = harness.state_mut();
+
+    // Mock the fractal clock time so snapshots are consistent.
+    app.state.clock.mock_time = Some(36383.0);
+
+    let apps = app
+        .apps_iter_mut()
+        .map(|(name, anchor, _)| (name, anchor))
+        .collect::<Vec<_>>();
+
+    #[cfg(feature = "wgpu")]
+    assert!(
+        apps.iter()
+            .any(|(_, anchor)| matches!(anchor, Anchor::Custom3d)),
+        "Expected to find the Custom3d app.",
+    );
+
+    let mut results = vec![];
+
+    for (name, anchor) in apps {
+        harness.get_by_role_and_label(Role::Button, name).click();
+
+        match anchor {
+            // The widget gallery demo shows the current date, so we can't use it for snapshot testing
+            Anchor::Demo => {
+                continue;
+            }
+            // This is already tested extensively elsewhere
+            Anchor::Rendering => {
+                continue;
+            }
+            // We don't want to rely on a network connection for tests
+            #[cfg(feature = "http")]
+            Anchor::Http => {
+                continue;
+            }
+            // Load a local image where we know it exists and loads quickly
+            #[cfg(feature = "image_viewer")]
+            Anchor::ImageViewer => {
+                harness.run();
+
+                harness
+                    .get_by_role_and_label(Role::TextInput, "URI:")
+                    .focus();
+                harness.press_key_modifiers(egui::Modifiers::COMMAND, egui::Key::A);
+
+                harness
+                    .get_by_role_and_label(Role::TextInput, "URI:")
+                    .type_text("file://../eframe/data/icon.png");
+
+                harness.get_by_role_and_label(Role::Button, "✔").click();
+            }
+            _ => {}
+        }
+
+        harness.run();
+
+        if let Err(e) = harness.try_snapshot(&anchor.to_string()) {
+            results.push(e);
+        }
+    }
+
+    if let Some(error) = results.first() {
+        panic!("{error}");
+    }
+}