Add regression test for QuerySets from get_queryset being off

aleksis/core/tests/regression/test_regression.py

@@ -19,3 +19,17 @@ def test_all_settigns_registered():
         )
 
         assert in_site_reg != in_person_reg
+
+
+def test_custom_managers_return_correct_qs():
+    """Tests that custom managers' get_queryset methods return the expected qs.
+
+    https://edugit.org/AlekSIS/official/AlekSIS-Core/-/issues/594
+    """
+
+    from aleksis.core import managers
+
+    def _check_get_queryset(Manager, QuerySet):
+        assert isinstance(Manager.from_queryset(QuerySet)().get_queryset(), QuerySet)
+
+    _check_get_queryset(managers.GroupManager, managers.GroupQuerySet)