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

[dcharkes]

While reviewing #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

[liamappelbe]

I've fixed all the flakes I could find, but there are some that I can't repro locally. I've done my best to fix those too, but I might have missed some. Please reopen or file another issue if there are more flakes on CI.

[liamappelbe]

global_test and global_native_test are still flaky. Can't repro locally.

https://github.com/dart-lang/native/actions/runs/10605979638/job/29397792224