Test for migration 0071 flaky

[luisa-beerboom]

I've recently noticed that full tests sometime fail at migration 71.
Rerunning fixed it despite there being no code changes.
That's why I'm assuming it's the tests fault.

I've not been able to reproduce this using single file tests, which is doubly annoying, however it might have something to do with the order in which the new submitters are created, based on the assertion errors I am vaguely remembering.

Considering how long the backend tests run, this ought to be investigated sooner rather than later.

[luisa-beerboom]

Error message is

        finalize("0071_new_motion_supporter_model")
    
>       assert_model(
            "meeting/1",
            {
                "id": 1,
                "motion_ids": [101, 102],
                "meeting_user_ids": [11, 21, 31],
                "motion_supporter_ids": [1, 2, 3, 4],
            },
        )

tests/system/migrations/test_0071_new_motion_supporter_model.py:168: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
tests/system/migrations/conftest.py:192: in _assert_model
    compare_models(model, expected)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

model = {'id': 1, 'meeting_user_ids': [11, 21, 31], 'meta_deleted': False, 'meta_position': 3, ...}
expected = {'id': 1, 'meeting_user_ids': [11, 21, 31], 'meta_deleted': False, 'meta_position': 3, ...}

    def compare_models(model, expected):
        # fix order of lists
        for key, value in expected.items():
            if (
                key in model
                and isinstance(model[key], list)
                and isinstance(value, list)
                and sorted(model[key]) == sorted(value)
            ):
                expected[key] = model[key]
>       assert model == expected
E       AssertionError: assert {'id': 1, 'me...tion': 3, ...} == {'id': 1, 'me...tion': 3, ...}
E         
E         Omitting 5 identical items, use -vv to show
E         Differing items:
E         {'motion_supporter_ids': [2, 3, 4, 5]} != {'motion_supporter_ids': [1, 2, 3, 4]}
E         Use -v to get more diff

tests/system/migrations/conftest.py:165: AssertionError

Seems to me like the motions were handled in the opposite order.