Avoid error bounces when testing stream management

The test suite sends messages to the server JID while checking whether
the stream management code counts outgoing stanzas correctly.  We now
set type='headline' for those messages to avoid error bounces.

test/ejabberd_SUITE.erl

@@ -607,7 +607,9 @@ disco(Config) ->
 sm(Config) ->
     Server = ?config(server, Config),
     ServerJID = jid:make(<<"">>, Server, <<"">>),
-    Msg = #message{to = ServerJID, body = [#text{data = <<"body">>}]},
+    %% Send messages of type 'headline' so the server discards them silently
+    Msg = #message{to = ServerJID, type = headline,
+		   body = [#text{data = <<"body">>}]},
     true = ?config(sm, Config),
     %% Enable the session management with resumption enabled
     send(Config, #sm_enable{resume = true, xmlns = ?NS_STREAM_MGMT_3}),