Disable test that's causing bot to hang

This test causes the buildbot to hang. We don't currently have any hang
protection, which means this test ends up killing the bot. Disable the
test for now why we work on the root issues.

packages/flutter/test/widget/date_picker_test_disabled.dart

diff --git a/packages/flutter/test/widget/date_picker_test.dart b/packages/flutter/test/widget/date_picker_test_disabled.dart
similarity index 100%
rename from packages/flutter/test/widget/date_picker_test.dart
rename to packages/flutter/test/widget/date_picker_test_disabled.dart