[macOS] Delete FlutterCompositor tests (flutter#47527)

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: https://github.com/flutter/engine/blob/89e8de970cb99aa82e067bbdb4a8e927e53f0b28/shell/platform/darwin/macos/framework/Source/FlutterCompositorTest.mm#L107-L137

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:
https://github.com/flutter/engine/blob/89e8de970cb99aa82e067bbdb4a8e927e53f0b28/shell/platform/darwin/macos/framework/Source/FlutterCompositorTest.mm#L47-L85

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

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

ci/licenses_golden/licenses_flutter

@@ -4669,7 +4669,6 @@ ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterCha
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterChannelKeyResponderTest.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterCompositor.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterCompositor.mm + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterCompositorTest.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterDartProject.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterDartProject_Internal.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterEmbedderExternalTextureTest.mm + ../../../flutter/LICENSE
@@ -7473,7 +7472,6 @@ FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterChann
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterChannelKeyResponderTest.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterCompositor.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterCompositor.mm
-FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterCompositorTest.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterDartProject.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterDartProject_Internal.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterEmbedderExternalTextureTest.mm

shell/platform/darwin/macos/BUILD.gn

@@ -173,7 +173,6 @@ executable("flutter_desktop_darwin_unittests") {
     "framework/Source/FlutterAppDelegateTest.mm",
     "framework/Source/FlutterAppLifecycleDelegateTest.mm",
     "framework/Source/FlutterChannelKeyResponderTest.mm",
-    "framework/Source/FlutterCompositorTest.mm",
     "framework/Source/FlutterEmbedderExternalTextureTest.mm",
     "framework/Source/FlutterEmbedderKeyResponderTest.mm",
     "framework/Source/FlutterEngineTest.mm",

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

@@ -20,6 +20,8 @@ namespace flutter {
 // FlutterCompositor creates and manages the backing stores used for
 // rendering Flutter content and presents Flutter content and Platform views.
 // Platform views are not yet supported.
+//
+// TODO(cbracken): refactor for testability. https://github.com/flutter/flutter/issues/137648
 class FlutterCompositor {
  public:
   // Create a FlutterCompositor with a view provider.