fix: test cases

Author: irtazaakram

xmodule/modulestore/__init__.py

@@ -15,6 +15,7 @@
 from zoneinfo import ZoneInfo
 
 from django.db import transaction
+from django.conf import settings
 from opaque_keys.edx.keys import AssetKey, CourseKey
 from opaque_keys.edx.locations import Location  # For import backwards compatibility
 from sortedcontainers import SortedKeyList
@@ -1196,7 +1197,8 @@ def __init__(  # lint-amnesty, pylint: disable=unused-argument
         # TODO move the inheritance_cache_subsystem to classes which use it
         self.metadata_inheritance_cache_subsystem = metadata_inheritance_cache_subsystem
         self.request_cache = request_cache
-        self.xblock_mixins = xblock_mixins
+        settings_mixins = getattr(settings, 'XBLOCK_MIXINS', ())
+        self.xblock_mixins = tuple(dict.fromkeys(settings_mixins + xblock_mixins))
         self.xblock_select = xblock_select
         self.xblock_field_data_wrappers = xblock_field_data_wrappers
         self.disabled_xblock_types = disabled_xblock_types