[macOS] Disable FlutterEngineTest.CanOverrideBackgroundColor

This was re-enabled in flutter/flutter#124677
after fixing leaks in #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: flutter/flutter#145728
Related issue: flutter/flutter#124677

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

@@ -251,7 +251,7 @@ @implementation MockableFlutterEngine
   latch.Wait();
 }
 
-TEST_F(FlutterEngineTest, CanOverrideBackgroundColor) {
+TEST_F(FlutterEngineTest, DISABLED_CanOverrideBackgroundColor) {
   FlutterEngine* engine = GetFlutterEngine();
 
   // Latch to ensure the entire layer tree has been generated and presented.