[macOS] Delete FlutterCompositor tests

[cbracken]

The tests for FlutterCompositor are not useful. The current tests test two things:

1. That the mocks we set up behave the way we set them up to behave.
2. That the implementation of FlutterCompositor is the current implementation of FlutterCompositor.

As an example, consider FlutterCompositorTest.TestPresent:

engine/shell/platform/darwin/macos/framework/Source/FlutterCompositorTest.mm

Lines 107 to 137 in 89e8de9

	TEST(FlutterCompositorTest, TestPresent) {
	__block NSArray<FlutterSurfacePresentInfo*>* presentedSurfaces = nil;
	auto onPresent = ^(NSArray<FlutterSurfacePresentInfo*>* info) {
	presentedSurfaces = info;
	};
	std::unique_ptr<flutter::FlutterCompositor> macos_compositor =
	std::make_unique<FlutterCompositor>(MockViewProvider(onPresent),
	/*platform_view_controller*/ nullptr);
	FlutterBackingStore backing_store;
	FlutterBackingStoreConfig config;
	config.struct_size = sizeof(FlutterBackingStoreConfig);
	config.size.width = 800;
	config.size.height = 600;
	macos_compositor->CreateBackingStore(&config, &backing_store);
	FlutterLayer layers[] = {{
	.struct_size = sizeof(FlutterLayer),
	.type = kFlutterLayerContentTypeBackingStore,
	.backing_store = &backing_store,
	.offset = {0, 0},
	.size = {800, 600},
	}};
	const FlutterLayer* layers_ptr = layers;
	macos_compositor->Present(kFlutterImplicitViewId, &layers_ptr, 1);
	ASSERT_EQ(presentedSurfaces.count, 1ul);
	}

Ostensibly, this test verifies that the onPresent callback configured in our fake FlutterViewProvider implementation is called when FlutterCompositor::Present() is called.

However, taking a look at the mocking setup:

engine/shell/platform/darwin/macos/framework/Source/FlutterCompositorTest.mm

Lines 47 to 85 in 89e8de9

	id<FlutterViewProvider> MockViewProvider(PresentBlock onPresent = nil) {
	FlutterView* viewMock = OCMClassMock([FlutterView class]);
	FlutterSurfaceManager* surfaceManagerMock = OCMClassMock([FlutterSurfaceManager class]);
	FlutterSurface* surfaceMock = OCMClassMock([FlutterSurface class]);
	__block id<MTLTexture> textureMock = OCMProtocolMock(@protocol(MTLTexture));
	OCMStub([viewMock surfaceManager]).andReturn(surfaceManagerMock);
	OCMStub([surfaceManagerMock surfaceForSize:CGSize{}])
	.ignoringNonObjectArgs()
	.andDo(^(NSInvocation* invocation) {
	CGSize size;
	[invocation getArgument:&size atIndex:2];
	OCMStub([textureMock width]).andReturn(size.width);
	OCMStub([textureMock height]).andReturn(size.height);
	})
	.andReturn(surfaceMock);
	FlutterMetalTexture texture = {
	.struct_size = sizeof(FlutterMetalTexture),
	.texture_id = 1,
	.texture = (__bridge void*)textureMock,
	.user_data = (__bridge void*)surfaceMock,
	.destruction_callback = nullptr,
	};
	OCMStub([surfaceManagerMock present:[OCMArg any] notify:[OCMArg any]])
	.andDo(^(NSInvocation* invocation) {
	NSArray<FlutterSurfacePresentInfo*>* info;
	[invocation getArgument:&info atIndex:2];
	if (onPresent != nil) {
	onPresent(info);
	}
	});
	OCMStub([surfaceMock asFlutterMetalTexture]).andReturn(texture);
	return [[FlutterViewMockProvider alloc] initWithImplicitView:viewMock];
	}

We do the following:

1. Mock out FlutterSurfaceManager such that when a surface is requested, we hand back a mock surface. A little gross since we're relying on some knowledge of implementation details of the compositor, but let's take this as reasonable for now.
2. We mock out FlutterSurface asFlutterMetalTexture to return a mock texture. Again, we're getting a bit deep into implementation details that the test shouldn't know about, but let's assume this gets us somewhere.
3. We mock out FlutterSurfaceManager present:notify: to actually call the onPresent callback if it's passed in.

In effect, we're testing that:

1. We configured our mock for FlutterSurfaceManager present:notify: to call onPresent.
2. That FlutterCompositor::Present actually calls FlutterSurfaceManager present:notify: despite that being a simple implementation detail of that call.

This removes these tests. I have filed the following issue to track refactoring this class for testability and adding tests: flutter/flutter#137648

Encountered these tests as part of deflaking and cleaning up memory allocations throughout the macOS desktop tests.

Issue: flutter/flutter#137648
Issue: flutter/flutter#104789
Issue: flutter/flutter#127441
Issue: flutter/flutter#124840

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[matanlurey]

LGTM I like this change

[goderbauer]

LGTM