[5.0.x] Fixed #34923 -- Avoided initializing LEVEL_TAGS when importing django.contrib.messages.

Regression in b7fe36ad37.

Thanks James Gillard for the report.

Backport of 311718feb5 from main

tests/messages_tests/tests.py

@@ -1,5 +1,6 @@
 from unittest import mock
 
+from django.conf import settings
 from django.contrib.messages import Message, add_message, constants
 from django.contrib.messages.storage import base
 from django.contrib.messages.test import MessagesTestMixin
@@ -63,6 +64,26 @@ class TestLevelTags(SimpleTestCase):
     def test_override_settings_level_tags(self):
         self.assertEqual(base.LEVEL_TAGS, self.message_tags)
 
+    def test_lazy(self):
+        # Don't use @override_settings to avoid calling the setting_changed
+        # signal.
+        old_message_tags = getattr(settings, "MESSAGE_TAGS", None)
+        settings.MESSAGE_TAGS = {constants.ERROR: "bad"}
+        try:
+            self.assertEqual(base.LEVEL_TAGS[constants.ERROR], "bad")
+        finally:
+            if old_message_tags is None:
+                del settings.MESSAGE_TAGS
+            else:
+                settings.MESSAGE_TAGS = old_message_tags
+
+    @override_settings(MESSAGE_TAGS=message_tags)
+    def test_override_settings_lazy(self):
+        # The update_level_tags handler has been called at least once before
+        # running this code when using @override_settings.
+        settings.MESSAGE_TAGS = {constants.ERROR: "very-bad"}
+        self.assertEqual(base.LEVEL_TAGS[constants.ERROR], "very-bad")
+
 
 class FakeResponse:
     def __init__(self):