[macOS] Remove extra setViewController and extra import (flutter#52255)

This stub PR removes and extra `setViewController`, which is unnecessary because the view controller's initializer already attaches itself to the engine.

This PR also removes an extra import since `FlutterCompositor` is not used in the engine's definition. As a result, `FlutterEngineTest.mm` needs to import it explicitly.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: dkwingsmt

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

@@ -20,6 +20,7 @@
 #import "flutter/shell/platform/darwin/macos/framework/Headers/FlutterAppDelegate.h"
 #import "flutter/shell/platform/darwin/macos/framework/Headers/FlutterAppLifecycleDelegate.h"
 #import "flutter/shell/platform/darwin/macos/framework/Headers/FlutterPluginMacOS.h"
+#import "flutter/shell/platform/darwin/macos/framework/Source/FlutterCompositor.h"
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterEngineTestUtils.h"
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterViewControllerTestUtils.h"
 #include "flutter/shell/platform/embedder/embedder.h"

shell/platform/darwin/macos/framework/Source/FlutterEngine_Internal.h

@@ -14,7 +14,6 @@
 #include "flutter/shell/platform/common/app_lifecycle_state.h"
 
 #import "flutter/shell/platform/darwin/macos/framework/Source/AccessibilityBridgeMac.h"
-#import "flutter/shell/platform/darwin/macos/framework/Source/FlutterCompositor.h"
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterPlatformViewController.h"
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterRenderer.h"
 

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

@@ -212,7 +212,6 @@ id MockGestureEvent(NSEventType type, NSEventPhase phase, double magnification,
                                                                                 nibName:nil
                                                                                  bundle:nil];
   [viewController loadView];
-  [engine setViewController:viewController];
   // Creates a NSWindow so that sub view can be first responder.
   NSWindow* window = [[NSWindow alloc] initWithContentRect:NSMakeRect(0, 0, 800, 600)
                                                  styleMask:NSBorderlessWindowMask