[ffigen] ref count test on Mac x64

[dcharkes]

@liamappelbe It looks like our ref count test failures on Mac x64 are back?

❌ test/native_objc_test/arc_test.dart: ARC retain properties ref count correctly (failed)
  Expected: <0>
    Actual: <2>
  
  package:matcher                            expect
  test/native_objc_test/arc_test.dart 364:7  main.<fn>.<fn>

https://github.com/dart-lang/native/actions/runs/20713316962/job/59460076224

[liamappelbe]

Yeah, it's a bit flaky. I can try adding a retry annotation to the test, though that didn't really help last time (the retries tended to all succeed or all fail).

[dcharkes]

If we can't make the test reliable, should we skip it on the CI by default?

[liamappelbe]

It's an important test though. I wonder if it's possible to add a retry to the entire workflow.