Move get_scheduler monkeypatching into an explicit pytest fixture

This allows us to actually run the lister instantiation code instead of relying
on the underlying structure of the lister object. In turn, this allows future
listers to use the scheduler right in their __init__.

swh/lister/core/tests/conftest.py

@@ -27,14 +27,12 @@ def lister_db_url(postgresql_proc, postgresql):
 
 
 @pytest.fixture
-def swh_listers(request, lister_db_url, swh_scheduler):
-
+def swh_listers(mock_get_scheduler, lister_db_url, swh_scheduler):
     listers = {}
 
     # Prepare schema for all listers
     for lister_name in SUPPORTED_LISTERS:
         lister = get_lister(lister_name, db_url=lister_db_url)
-        lister.scheduler = swh_scheduler  # inject scheduler fixture
         listers[lister_name] = lister
     initialize(create_engine(lister_db_url), drop_tables=True)
 

swh/lister/tests/test_cli.py

@@ -20,7 +20,7 @@ def test_get_lister_wrong_input():
     assert "Invalid lister" in str(e.value)
 
 
-def test_get_lister():
+def test_get_lister(mock_get_scheduler):
     """Instantiating a supported lister should be ok
 
     """