Mark xFail: List messages mentioning me test

This used to work. Now, it appears the APIs are not allowing or correctly handling self-referencing markdown mentions.

Author: cmlccie

tests/api/test_messages.py

@@ -285,6 +285,9 @@ def test_list_messages_before_message_with_id(
     assert are_valid_messages(message_list)
 
 
+@pytest.mark.xfail(
+    reason="This test is failing because the API is not returning the expected results"
+)
 def test_list_messages_mentioning_me(
     api, group_room, group_room_markdown_message
 ):