tests: add ui_is_suspended stub (upstream 3326ce5c6)

Add stub for ui_is_suspended() in tests/unittests/ui/stub_ui.c so
the unit-test binary links cleanly against any code path that calls
it from non-UI translation units.

tests/unittests/ui/stub_ui.c

@@ -1469,3 +1469,9 @@ cons_has_alerts(void)
 {
     return FALSE;
 }
+
+gboolean
+ui_is_suspended(void)
+{
+    return FALSE;
+}