test(disco): add functional test for disco#items error handling

Verifies that /disco items displays error responses (e.g. service-unavailable)
to the user, per XEP-0030 Section 7.

tests/functionaltests/test_disco.c

@@ -0,0 +1,36 @@
+#include <glib.h>
+#include <stdarg.h>
+#include <stddef.h>
+#include <setjmp.h>
+#include <cmocka.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <stabber.h>
+
+#include "proftest.h"
+
+void
+disco_items_error_handling(void **state)
+{
+    /*
+     * Test that /disco items displays error responses to user.
+     * Per XEP-0030 Section 7: errors like service-unavailable must be shown.
+     */
+    stbbr_for_query("http://jabber.org/protocol/disco#items",
+        "<iq to='stabber@localhost/profanity' type='error' from='broken.localhost'>"
+            "<error type='cancel'>"
+                "<service-unavailable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>"
+            "</error>"
+        "</iq>"
+    );
+
+    prof_connect();
+
+    prof_input("/disco items broken.localhost");
+
+    prof_timeout(10);
+    /* Per XEP-0030, error should be displayed to user */
+    assert_true(prof_output_regex("Service discovery failed.*service-unavailable"));
+    prof_timeout_reset();
+}