Date picker test is not hermetic

[mdebbar]

The tests in this repo run as part of Flutter's CI. It was discovered recently that one of the date picker tests wasn't hermetic. It fails whenever we run it on the first day of the month.

The test in question is: https://github.com/macosui/macos_ui/blob/b3a8e53c58b6503a4c03f26935f38895a7224180/test/selectors/date_picker_test.dart#L311

To reproduce the failure:

• Change the DateTime.now() to DateTime(2023, 12, 1).
• Run the test: flutter test test/selectors/date_picker_test.dart.

The tests from this repo will be disabled in Flutter CI until the issue is resolved.