Disable non-hermetic tests of macos_ui (#317)

Some tests in `macos_ui` are not hermetic. This PR disabled them in order to unblock the Flutter tree.

Issue filed: macosui/macos_ui#499

Author: mdebbar

registry/macos_ui.test

@@ -5,5 +5,10 @@ fetch=git -C tests checkout c6d37d88e38bddc0723fee9eb57c146aa9f0053d
 
 update=.
 
-test=flutter analyze --no-fatal-infos
-test=flutter test
+# Disable macos_ui tests due to them not being hermetic.
+# - Github issue: https://github.com/macosui/macos_ui/issues/499
+
+test=echo macos_ui tests are disabled
+
+#test=flutter analyze --no-fatal-infos
+#test=flutter test