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_kittest/src/builder.rs

@@ -166,6 +166,7 @@ impl<State> HarnessBuilder<State> {
     ///
     /// assert_eq!(*harness.state(), true);
     /// ```
+    #[track_caller]
     pub fn build_state<'a>(
         self,
         app: impl FnMut(&egui::Context, &mut State) + 'a,
@@ -195,6 +196,7 @@ impl<State> HarnessBuilder<State> {
     ///
     /// assert_eq!(*harness.state(), true);
     /// ```
+    #[track_caller]
     pub fn build_ui_state<'a>(
         self,
         app: impl FnMut(&mut egui::Ui, &mut State) + 'a,
@@ -206,6 +208,7 @@ impl<State> HarnessBuilder<State> {
     /// Create a new [Harness] from the given eframe creation closure.
     /// The app can be accessed via the [`Harness::state`] / [`Harness::state_mut`] methods.
     #[cfg(feature = "eframe")]
+    #[track_caller]
     pub fn build_eframe<'a>(
         self,
         build: impl FnOnce(&mut eframe::CreationContext<'a>) -> State,
@@ -247,6 +250,7 @@ impl HarnessBuilder {
     ///     });
     /// ```
     #[must_use]
+    #[track_caller]
     pub fn build<'a>(self, app: impl FnMut(&egui::Context) + 'a) -> Harness<'a> {
         Harness::from_builder(self, AppKind::Context(Box::new(app)), (), None)
     }
@@ -267,6 +271,7 @@ impl HarnessBuilder {
     ///     });
     /// ```
     #[must_use]
+    #[track_caller]
     pub fn build_ui<'a>(self, app: impl FnMut(&mut egui::Ui) + 'a) -> Harness<'a> {
         Harness::from_builder(self, AppKind::Ui(Box::new(app)), (), None)
     }