How to test a infinite while loop with pytest

I am currently writing a little lib that interacts with a bamboo build server. Testing is done using pytest. I got stuck at the following problem. I want to test a while loop that runs till some status is satisfied. Reading the pytest doc, I tried to "mock" / monkeypatch the status, but it doesnt really work. I am probably doing something elementary wrong here: This is the while loop in question:

    # determine current status
    running = self._is_a_build_running()

    # turn on and off running powerplug while building
    while running:
        self.feedback.turn_off_success()
        self.feedback.turn_on_running()
        time.sleep(self.blinker_time)
        self.feedback.turn_off_running()
        self._update_builds_status()
        running = self._is_a_build_running()

so what I tried with pytest was creating a fixture for a positive and a negative _is_a_build_running like this:

@pytest.fixture(scope='function')
def mock_is_a_build_running():
    return False

and then using this test method using a ThreadPool (explained here how to get the return value from a thread in python?) because I also would need a result from the method containing the while loop.

def test_update_status_running(bamboopickups, monkeypatch,
                   mock_update_overall_data_positive,
                   mock_update_builds_status_positive,
                   mock_is_a_build_running):
monkeypatch.setattr('BambooPickup._update_overall_data', lambda x: mock_update_overall_data_positive)
monkeypatch.setattr('BambooPickup._update_builds_status', lambda x: mock_update_builds_status_positive)

pool = ThreadPool(processes=1)
async_result = pool.apply_async(bamboopickups.update_status())

monkeypatch.setattr('BambooPickup._update_overall_data', lambda x: mock_update_overall_data_positive)
monkeypatch.setattr('BambooPickup._is_a_build_running', lambda x: mock_is_a_build_running)

actual = async_result.get()
expected = True
assert actual == expected

This is probably easily done with pytest-mock, but so far I was only using the prefered way described here: http://ift.tt/1wFjBXP.