Add Harness::spawn_eframe_app (#8120)

This lets you start up the test app from within the test itself, which
can be very useful when you have a specific test scenario set up that
you need to debug.

### Related
* Previous attempt: https://github.com/emilk/egui/pull/5418

### macOS
On macOS, you may only run UIs on the main loop, so you need a few
additional steps. Not ideal, but works!


```diff
diff --git a/crates/egui_demo_app/Cargo.toml b/crates/egui_demo_app/Cargo.toml
index f9a153268..4e0cc14ee 100644
--- a/crates/egui_demo_app/Cargo.toml
+++ b/crates/egui_demo_app/Cargo.toml
@@ -84,3 +84,7 @@ web-sys.workspace = true
 
 [dev-dependencies]
 egui_kittest = { workspace = true, features = ["eframe", "snapshot", "wgpu"] }
+
+[[test]]
+name = "test_demo_app"
+harness = false
diff --git a/crates/egui_demo_app/tests/test_demo_app.rs b/crates/egui_demo_app/tests/test_demo_app.rs
index e083c8455..7ad9ed516 100644
--- a/crates/egui_demo_app/tests/test_demo_app.rs
+++ b/crates/egui_demo_app/tests/test_demo_app.rs
@@ -4,7 +4,10 @@ use egui_demo_app::{Anchor, WrapApp};
 use egui_kittest::SnapshotResults;
 use egui_kittest::kittest::Queryable as _;
 
-#[test]
+fn main() {
+    test_demo_app();
+}
+
 fn test_demo_app() {
     let mut harness = egui_kittest::Harness::builder()
         .with_size(Vec2::new(900.0, 600.0))
@@ -73,5 +76,8 @@ fn test_demo_app() {
         harness.run_steps(4);
 
         results.add(harness.try_snapshot(anchor.to_string()));
+
+        harness.spawn_eframe_app();
+        break;
     }
 }
```

crates/egui_kittest/src/app_kind.rs

@@ -4,17 +4,21 @@ type AppKindUiState<'a, State> = Box<dyn FnMut(&mut egui::Ui, &mut State) + 'a>;
 type AppKindUi<'a> = Box<dyn FnMut(&mut egui::Ui) + 'a>;
 
 /// In order to access the [`eframe::App`] trait from the generic `State`, we store a function pointer
-/// here that will return the dyn trait from the struct. In the builder we have the correct where
-/// clause to be able to create this.
+/// here that will return the dyn trait from the struct.
+/// In the builder we have the correct `where`-clause to be able to create this.
 /// Later we can use it anywhere to get the [`eframe::App`] from the `State`.
 #[cfg(feature = "eframe")]
-type AppKindEframe<'a, State> = (fn(&mut State) -> &mut dyn eframe::App, eframe::Frame);
+pub(crate) struct AppKindEframe<State> {
+    pub get_app: fn(&mut State) -> &mut dyn eframe::App,
+    pub take_app: fn(State) -> Box<dyn eframe::App>,
+    pub frame: eframe::Frame,
+}
 
 pub(crate) enum AppKind<'a, State> {
     Ui(AppKindUi<'a>),
     UiState(AppKindUiState<'a, State>),
     #[cfg(feature = "eframe")]
-    Eframe(AppKindEframe<'a, State>),
+    Eframe(AppKindEframe<State>),
 }
 
 impl<State> AppKind<'_, State> {
@@ -26,13 +30,10 @@ impl<State> AppKind<'_, State> {
     ) -> Option<egui::Response> {
         match self {
             #[cfg(feature = "eframe")]
-            AppKind::Eframe((get_app, frame)) => {
+            AppKind::Eframe(AppKindEframe { get_app, frame, .. }) => {
                 let app = get_app(state);
-
                 app.logic(ui, frame);
-
                 app.ui(ui, frame);
-
                 None
             }
             kind_ui => Some(kind_ui.run_ui(ui, state, sizing_pass)),