Enforce consistent snapshot updates (#7744)

* Closes https://github.com/emilk/egui/issues/7647


This collects SnapshotResults within the Harness and adds a check to
enforce snapshot results are merged in case multiple Harnesses are
constructed within a test.

This should make snapshot updates via kitdiff/accept_snapshots.sh way
more useful since it should now always update all snapshots instead of
only the first one per test.

crates/egui_demo_lib/src/demo/tests/tessellation_test.rs

@@ -357,11 +357,13 @@ fn rect_shape_ui(ui: &mut egui::Ui, shape: &mut RectShape) {
 #[cfg(test)]
 mod tests {
     use crate::View as _;
+    use egui_kittest::SnapshotResults;
 
     use super::*;
 
     #[test]
     fn snapshot_tessellation_test() {
+        let mut results = SnapshotResults::new();
         for (name, shape) in TessellationTest::interesting_shapes() {
             let mut test = TessellationTest {
                 shape,
@@ -375,6 +377,7 @@ mod tests {
             harness.run();
 
             harness.snapshot(format!("tessellation_test/{name}"));
+            results.extend_harness(&mut harness);
         }
     }
 }

crates/egui_demo_lib/src/demo/widget_gallery.rs

@@ -310,7 +310,7 @@ mod tests {
     use super::*;
     use crate::View as _;
     use egui::Vec2;
-    use egui_kittest::Harness;
+    use egui_kittest::{Harness, SnapshotResults};
 
     #[test]
     pub fn should_match_screenshot() {
@@ -320,6 +320,8 @@ mod tests {
             ..Default::default()
         };
 
+        let mut results = SnapshotResults::new();
+
         for pixels_per_point in [1, 2] {
             for theme in [egui::Theme::Light, egui::Theme::Dark] {
                 let mut harness = Harness::builder()
@@ -339,6 +341,7 @@ mod tests {
                 };
                 let image_name = format!("widget_gallery_{theme_name}_x{pixels_per_point}");
                 harness.snapshot(&image_name);
+                results.extend_harness(&mut harness);
             }
         }
     }