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__.

conftest.py

@@ -5,6 +5,24 @@
 
 import os
 
+import pytest
+
 pytest_plugins = ["swh.scheduler.pytest_plugin"]
 
 os.environ["LC_ALL"] = "C.UTF-8"
+
+
+@pytest.fixture
+def mock_get_scheduler(monkeypatch, swh_scheduler):
+    """Override the get_scheduler function in swh.lister.core.lister_base, to
+    return the swh_scheduler fixture.
+    """
+    from swh.lister.core import lister_base
+
+    # Match the signature from swh.scheduler.get_scheduler
+    def get_scheduler(cls, args={}):
+        return swh_scheduler
+
+    monkeypatch.setattr(lister_base, "get_scheduler", get_scheduler)
+
+    yield monkeypatch