[macOS] Replace fixture subclasses with usings (#48111)

In cases where we subclassed gtest fixtures purely to get better naming for test output (i.e. displaying/filtering on AccessibilityBridgeMacTest instead of AutoreleasePoolTest, replace these with `using` declarations, which is equally effective and avoids actually subclassing.

This applies no semantic changes to the code or tests.

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

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

@@ -80,18 +80,11 @@ @implementation AccessibilityBridgeTestViewController
   FML_DISALLOW_COPY_AND_ASSIGN(AccessibilityBridgeMacWindowTest);
 };
 
-// AutoreleasePoolTest subclass that exists simply to provide more specific naming.
-class AccessibilityBridgeMacTest : public AutoreleasePoolTest {
- public:
-  AccessibilityBridgeMacTest() = default;
-  ~AccessibilityBridgeMacTest() = default;
-
- private:
-  FML_DISALLOW_COPY_AND_ASSIGN(AccessibilityBridgeMacTest);
-};
-
 NSWindow* AccessibilityBridgeMacWindowTest::gWindow_ = nil;
 
+// Test-specific name for AutoreleasePoolTest fixture.
+using AccessibilityBridgeMacTest = AutoreleasePoolTest;
+
 }  // namespace
 
 TEST_F(AccessibilityBridgeMacWindowTest, SendsAccessibilityCreateNotificationFlutterViewWindow) {

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

@@ -67,15 +67,8 @@ - (CVPixelBufferRef)pixelBuffer {
 
 namespace flutter::testing {
 
-// AutoreleasePoolTest subclass that exists simply to provide more specific naming.
-class FlutterEmbedderExternalTextureTest : public AutoreleasePoolTest {
- public:
-  FlutterEmbedderExternalTextureTest() = default;
-  ~FlutterEmbedderExternalTextureTest() = default;
-
- private:
-  FML_DISALLOW_COPY_AND_ASSIGN(FlutterEmbedderExternalTextureTest);
-};
+// Test-specific name for AutoreleasePoolTest fixture.
+using FlutterEmbedderExternalTextureTest = AutoreleasePoolTest;
 
 TEST_F(FlutterEmbedderExternalTextureTest, TestTextureResolution) {
   // Constants.

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

@@ -174,25 +174,9 @@ id MockGestureEvent(NSEventType type, NSEventPhase phase, double magnification,
 
 #pragma mark - gtest tests
 
-// AutoreleasePoolTest subclass that exists simply to provide more specific naming.
-class FlutterViewControllerTest : public AutoreleasePoolTest {
- public:
-  FlutterViewControllerTest() = default;
-  ~FlutterViewControllerTest() = default;
-
- private:
-  FML_DISALLOW_COPY_AND_ASSIGN(FlutterViewControllerTest);
-};
-
-// MockFlutterEngineTest subclass that exists simply to provide more specific naming.
-class FlutterViewControllerMockEngineTest : public MockFlutterEngineTest {
- public:
-  FlutterViewControllerMockEngineTest() = default;
-  ~FlutterViewControllerMockEngineTest() = default;
-
- private:
-  FML_DISALLOW_COPY_AND_ASSIGN(FlutterViewControllerMockEngineTest);
-};
+// Test-specific names for AutoreleasePoolTest, MockFlutterEngineTest fixtures.
+using FlutterViewControllerTest = AutoreleasePoolTest;
+using FlutterViewControllerMockEngineTest = MockFlutterEngineTest;
 
 TEST_F(FlutterViewControllerTest, HasViewThatHidesOtherViewsInAccessibility) {
   FlutterViewController* viewControllerMock = CreateMockViewController();