[macOS] Disable FlutterEngineTest.BackgroundColor (#52341)

This was re-enabled in https://github.com/flutter/flutter/issues/124677 after fixing leaks in https://github.com/flutter/engine/pull/50832. Since this test involves an NSWindow, there are separate fixes that should be landed to reduce flakes related to lack of NSWindow close calls in the tests. See related AppKit bug report filed at http://www.openradar.me/FB13291861.

Issue: https://github.com/flutter/flutter/issues/145728
Related issue: https://github.com/flutter/flutter/issues/124677

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

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

diff --git a/shell/platform/darwin/macos/framework/Source/FlutterEngineTest.mm b/shell/platform/darwin/macos/framework/Source/FlutterEngineTest.mm
index ffb4d9b..8bd661e 100644
--- a/shell/platform/darwin/macos/framework/Source/FlutterEngineTest.mm
+++ b/shell/platform/darwin/macos/framework/Source/FlutterEngineTest.mm
@@ -223,7 +223,7 @@
   EXPECT_TRUE(stdout_capture.GetOutput().find("Hello logging") != std::string::npos);
 }
 
-TEST_F(FlutterEngineTest, BackgroundIsBlack) {
+TEST_F(FlutterEngineTest, DISABLED_BackgroundIsBlack) {
   FlutterEngine* engine = GetFlutterEngine();
 
   // Latch to ensure the entire layer tree has been generated and presented.