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/eframe/src/native/glow_integration.rs

@@ -55,6 +55,10 @@ pub struct GlowWinitApp<'app> {
     // re-initializing the `GlowWinitRunning` state on Android if the application
     // suspends and resumes.
     app_creator: Option<AppCreator<'app>>,
+
+    /// An optional pre-existing egui context. If `Some`, it is used instead of
+    /// creating a new one via [`create_egui_context`]. Taken during initialization.
+    egui_ctx: Option<egui::Context>,
 }
 
 /// State that is initialized when the application is first starts running via
@@ -128,6 +132,7 @@ impl<'app> GlowWinitApp<'app> {
         event_loop: &EventLoop<UserEvent>,
         app_name: &str,
         native_options: NativeOptions,
+        egui_ctx: Option<egui::Context>,
         app_creator: AppCreator<'app>,
     ) -> Self {
         profiling::function_scope!();
@@ -137,6 +142,7 @@ impl<'app> GlowWinitApp<'app> {
             native_options,
             running: None,
             app_creator: Some(app_creator),
+            egui_ctx,
         }
     }
 
@@ -209,7 +215,10 @@ impl<'app> GlowWinitApp<'app> {
             )
         };
 
-        let egui_ctx = create_egui_context(storage.as_deref());
+        let egui_ctx = self
+            .egui_ctx
+            .take()
+            .unwrap_or_else(|| create_egui_context(storage.as_deref()));
 
         let (mut glutin, painter) = Self::create_glutin_windowed_context(
             &egui_ctx,