test(autoping): add test for autoping warning exclusion on subdomain services

The new test verifies that disco#info responses from subdomain services
(e.g., conference servers) do not trigger the autoping warning, ensuring
the check is correctly restricted to the server's own domain.

tests/functionaltests/test_autoping.h

@@ -9,3 +9,4 @@ void autoping_warning_not_shown_when_server_unsupported(void** state);
 void autoping_warning_not_shown_when_autoping_enabled(void** state);
 void autoping_warning_not_shown_when_user_disabled(void** state);
 void autoping_warning_not_shown_for_user_jid(void** state);
+void autoping_warning_not_shown_for_subdomain_service(void** state);