[ffigen][objective_c] test/native_objc_test/automated_ref_count_test.dart flaky

[dcharkes]

While reviewing https://github.com/dart-lang/native/pull/1434/:

00:03 +13 -1: test/native_objc_test/automated_ref_count_test.dart: Automatic reference counting assign properties ref count correctly [E]                                                                                                                                                   
  Expected: <1>
    Actual: <0>

After rerunning the test passed.

cc @liamappelbe