Flaky test in swh-graph

The following swh-graph test often ends up flaky, see swh-environment Jenkins job builds history.

python3 -m pytest  .
============================= test session starts ==============================
platform linux -- Python 3.7.3, pytest-6.2.5, py-1.11.0, pluggy-1.0.0
rootdir: /home/jenkins/workspace/DENV/tests/swh-graph, configfile: pytest.ini
plugins: asyncio-0.16.0, hypothesis-6.34.1, dash-2.0.0, mock-3.6.1, django-test-migrations-1.2.0, xdist-2.5.0, django-4.5.2, postgresql-2.6.1, flask-1.2.0, requests-mock-1.9.3, redis-2.3.0, forked-1.4.0, swh.core-1.0.1.dev1+g4ff374f, swh.journal-0.9.2.dev2+g4e5e009
collected 43 items

swh/graph/tests/test_api_client.py .........sF...s.........s....s        [ 69%]
swh/graph/tests/test_cli.py .                                            [ 72%]
swh/graph/tests/test_swhid.py ............                               [100%]

=================================== FAILURES ===================================
_____________________ test_random_walk_dst_is_type[remote] _____________________

graph_client = <RemoteGraphClient url=http://127.0.0.1:39123/graph/>

    def test_random_walk_dst_is_type(graph_client):
        """as the walk is random, we test a visit from a cnt node to the only
        origin in the dataset, and only check the final node of the path
        (i.e., the origin)
        """
        args = ("swh:1:cnt:0000000000000000000000000000000000000001", "ori")
        kwargs = {"direction": "backward"}
        expected_root = "swh:1:ori:0000000000000000000000000000000000000021"
    
        actual = list(graph_client.random_walk(*args, **kwargs))
        assert len(actual) > 1  # no origin directly links to a content
        assert actual[0] == args[0]
        assert actual[-1] == expected_root
    
        kwargs2 = kwargs.copy()
        kwargs2["limit"] = -1
        actual = list(graph_client.random_walk(*args, **kwargs2))
        assert actual == [expected_root]
    
        kwargs2["limit"] = -2
        actual = list(graph_client.random_walk(*args, **kwargs2))
        assert len(actual) == 2
        assert actual[-1] == expected_root
    
        kwargs2["limit"] = 3
        actual = list(graph_client.random_walk(*args, **kwargs2))
>       assert len(actual) == 3
E       AssertionError: assert 1 == 3
E        +  where 1 = len([''])

swh/graph/tests/test_api_client.py:281: AssertionError
=========================== short test summary info ============================
FAILED swh/graph/tests/test_api_client.py::test_random_walk_dst_is_type[remote]
=================== 1 failed, 38 passed, 4 skipped in 8.69s ====================
make: *** [../Makefile.python:20: test] Error 1

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Migrated from T3831 (view on Phabricator)