[macOS] Fix test fixture identifier style (flutter#33118)

Rename the C++ style can_composite_platform_views and native_callback
functions to the Dart-style identifiers canCompositePlatformViews and
nativeCallback in the macOS desktop test fixtures.

Also corrects the helper method identifier from CreateSimplePicture to
_createSimplePicture, thus making it library-private to help readers
understand it's a local helper.

Ref: [Flutter Style Guide](https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#naming)
Ref: [Effective Dart](https://dart.dev/guides/language/effective-dart/style#do-name-other-identifiers-using-lowercamelcase)

Author: cbracken

shell/platform/darwin/macos/framework/Source/FlutterEngineTest.mm

@@ -357,7 +357,7 @@ @interface FlutterEngine (Test)
 
 TEST_F(FlutterEngineTest, NativeCallbacks) {
   FlutterEngine* engine = GetFlutterEngine();
-  EXPECT_TRUE([engine runWithEntrypoint:@"native_callback"]);
+  EXPECT_TRUE([engine runWithEntrypoint:@"nativeCallback"]);
   EXPECT_TRUE(engine.running);
 
   fml::AutoResetWaitableEvent latch;
@@ -384,7 +384,7 @@ @interface FlutterEngine (Test)
   viewController.flutterView.frame = CGRectMake(0, 0, 800, 600);
   [engine setViewController:viewController];
 
-  EXPECT_TRUE([engine runWithEntrypoint:@"can_composite_platform_views"]);
+  EXPECT_TRUE([engine runWithEntrypoint:@"canCompositePlatformViews"]);
 
   // Latch to ensure the entire layer tree has been generated and presented.
   fml::AutoResetWaitableEvent latch;

shell/platform/darwin/macos/framework/Source/fixtures/flutter_desktop_test.dart

@@ -16,29 +16,30 @@ void canLogToStdout() {
   signalNativeTest();
 }
 
-Picture CreateSimplePicture() {
-  Paint blackPaint = Paint();
-  PictureRecorder baseRecorder = PictureRecorder();
-  Canvas canvas = Canvas(baseRecorder);
-  canvas.drawRect(Rect.fromLTRB(0.0, 0.0, 1000.0, 1000.0), blackPaint);
-  return baseRecorder.endRecording();
-}
-
 @pragma('vm:entry-point')
-void can_composite_platform_views() {
+void canCompositePlatformViews() {
   PlatformDispatcher.instance.onBeginFrame = (Duration duration) {
     SceneBuilder builder = SceneBuilder();
-    builder.addPicture(Offset(1.0, 1.0), CreateSimplePicture());
+    builder.addPicture(Offset(1.0, 1.0), _createSimplePicture());
     builder.pushOffset(1.0, 2.0);
     builder.addPlatformView(42, width: 123.0, height: 456.0);
-    builder.addPicture(Offset(1.0, 1.0), CreateSimplePicture());
+    builder.addPicture(Offset(1.0, 1.0), _createSimplePicture());
     builder.pop(); // offset
     PlatformDispatcher.instance.views.first.render(builder.build());
   };
   PlatformDispatcher.instance.scheduleFrame();
 }
 
+/// Returns a [Picture] of a simple black square.
+Picture _createSimplePicture() {
+  Paint blackPaint = Paint();
+  PictureRecorder baseRecorder = PictureRecorder();
+  Canvas canvas = Canvas(baseRecorder);
+  canvas.drawRect(Rect.fromLTRB(0.0, 0.0, 1000.0, 1000.0), blackPaint);
+  return baseRecorder.endRecording();
+}
+
 @pragma('vm:entry-point')
-void native_callback() {
+void nativeCallback() {
   signalNativeTest();
 }