BUG: TestProgress::test_progress is flaky due to race condition

While building on nixpkgs' CI ("Hydra"), test_progress fails intermittently:

=================================== FAILURES ===================================
__________________________ TestProgress.test_progress __________________________

self = <tests.test_progress.TestProgress object at 0x1104830b0>

    def test_progress(self):
        seen_preeval = False
        seen_eval = False
        seen_posteval = False
    
        def preeval_cb(image, progress):
            nonlocal seen_preeval
            seen_preeval = True
    
        def eval_cb(image, progress):
            nonlocal seen_eval
            seen_eval = True
    
        def posteval_cb(image, progress):
            nonlocal seen_posteval
            seen_posteval = True
    
        image = pyvips.Image.black(1, 100000)
        image.set_progress(True)
        image.signal_connect('preeval', preeval_cb)
        image.signal_connect('eval', eval_cb)
        image.signal_connect('posteval', posteval_cb)
        image.avg()
    
        assert seen_preeval
>       assert seen_eval
E       assert False

tests/test_progress.py:34: AssertionError

Suggested fix: Mark as flaky.

Start in tests/test_progress.py at TestProgress.test_progress and review the intermittent assertion failure shown in the issue. Apply the suggested flaky-test marking, then run the targeted test and verify that the test suite or CI no longer treats this race as a failure.