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/builder.rs

@@ -1,4 +1,6 @@
 use crate::app_kind::AppKind;
+#[cfg(feature = "eframe")]
+use crate::app_kind::AppKindEframe;
 use crate::{Harness, LazyRenderer, TestRenderer};
 use egui::{Pos2, Rect, Vec2};
 use std::marker::PhantomData;
@@ -196,7 +198,7 @@ impl<State> HarnessBuilder<State> {
         build: impl FnOnce(&mut eframe::CreationContext<'a>) -> State,
     ) -> Harness<'a, State>
     where
-        State: eframe::App,
+        State: eframe::App + 'static,
     {
         let ctx = egui::Context::default();
 
@@ -207,7 +209,11 @@ impl<State> HarnessBuilder<State> {
 
         let app = build(&mut cc);
 
-        let kind = AppKind::Eframe((|state| state, frame));
+        let kind = AppKind::Eframe(AppKindEframe {
+            get_app: |state| state,
+            take_app: |state| Box::new(state),
+            frame,
+        });
         Harness::from_builder(self, kind, app, Some(ctx))
     }
 }