Fixed #34730 -- Added django.contrib.messages.test.MessagesTestMixin.assertMessages().

tests/messages_tests/utils.py

@@ -0,0 +1,14 @@
+from django.contrib.messages import Message
+
+
+class DummyStorage:
+    """Dummy message-store to test the API methods."""
+
+    def __init__(self):
+        self.store = []
+
+    def add(self, level, message, extra_tags=""):
+        self.store.append(Message(level, message, extra_tags))
+
+    def __iter__(self):
+        return iter(self.store)