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/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
 
     """