Add ImageLoader::has_pending and wait_for_pending_images (#7030)

With kittest it was difficult to wait for images to be loaded before
taking a snapshot test.
This PR adds `Harness::with_wait_for_pending_images` (true by default)
which will cause `Harness::run` to sleep until all images are loaded (or
`HarnessBuilder::with_max_steps` is exceeded).

It also adds a new ImageLoader::has_pending and
BytesLoader::has_pending, which should be implemented if things are
loaded / decoded asynchronously.

It reverts https://github.com/emilk/egui/pull/6901 which was my previous
attempt to fix this (but this didn't work since only the tested crate is
compiled with cfg(test) and not it's dependencies)

crates/egui_extras/src/loaders/ehttp_loader.rs

@@ -125,4 +125,8 @@ impl BytesLoader for EhttpLoader {
             })
             .sum()
     }
+
+    fn has_pending(&self) -> bool {
+        self.cache.lock().values().any(|entry| entry.is_pending())
+    }
 }

crates/egui_extras/src/loaders/file_loader.rs

@@ -128,4 +128,8 @@ impl BytesLoader for FileLoader {
             })
             .sum()
     }
+
+    fn has_pending(&self) -> bool {
+        self.cache.lock().values().any(|entry| entry.is_pending())
+    }
 }

crates/egui_extras/src/loaders/image_loader.rs

@@ -8,6 +8,9 @@ use egui::{
 use image::ImageFormat;
 use std::{mem::size_of, path::Path, sync::Arc, task::Poll};
 
+#[cfg(not(target_arch = "wasm32"))]
+use std::thread;
+
 type Entry = Poll<Result<Arc<ColorImage>, String>>;
 
 #[derive(Default)]
@@ -73,7 +76,7 @@ impl ImageLoader for ImageCrateLoader {
             return Err(LoadError::NotSupported);
         }
 
-        #[cfg(not(any(target_arch = "wasm32", test)))]
+        #[cfg(not(target_arch = "wasm32"))]
         #[expect(clippy::unnecessary_wraps)] // needed here to match other return types
         fn load_image(
             ctx: &egui::Context,
@@ -85,7 +88,7 @@ impl ImageLoader for ImageCrateLoader {
             cache.lock().insert(uri.clone(), Poll::Pending);
 
             // Do the image parsing on a bg thread
-            std::thread::Builder::new()
+            thread::Builder::new()
                 .name(format!("egui_extras::ImageLoader::load({uri:?})"))
                 .spawn({
                     let ctx = ctx.clone();
@@ -113,8 +116,7 @@ impl ImageLoader for ImageCrateLoader {
             Ok(ImagePoll::Pending { size: None })
         }
 
-        // Load images on the current thread for tests, so they are less flaky
-        #[cfg(any(target_arch = "wasm32", test))]
+        #[cfg(target_arch = "wasm32")]
         fn load_image(
             _ctx: &egui::Context,
             uri: &str,
@@ -179,6 +181,10 @@ impl ImageLoader for ImageCrateLoader {
             })
             .sum()
     }
+
+    fn has_pending(&self) -> bool {
+        self.cache.lock().values().any(|result| result.is_pending())
+    }
 }
 
 #[cfg(test)]