Add functional tests for /disco command (XEP-0030)

- Add 8 tests for disco info and disco items commands
- Fix XEP-0030 compliance bug: show message for empty disco#items results
- Tests cover: identity display, features, server/jid queries, error handling,
  items display, empty results, and connection requirement

tests/functionaltests/test_disco.h

@@ -0,0 +1,13 @@
+/* test_disco.h
+ *
+ * Functional tests for /disco command (XEP-0030 Service Discovery)
+ */
+
+void disco_info_shows_identity(void **state);
+void disco_info_shows_features(void **state);
+void disco_info_to_server(void **state);
+void disco_info_to_jid(void **state);
+void disco_info_not_found(void **state);
+void disco_items_shows_items(void **state);
+void disco_items_empty_result(void **state);
+void disco_requires_connection(void **state);