tests/kms: Don't leak MetaKmsFeedback

Also start warning if we don't handle the
meta_kms_device_process_update_sync() return value.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2853>

src/backends/native/meta-kms-device.h

@@ -75,7 +75,8 @@ GList * meta_kms_device_get_fallback_modes (MetaKmsDevice *device);
 META_EXPORT_TEST
 MetaKmsFeedback * meta_kms_device_process_update_sync (MetaKmsDevice     *device,
                                                        MetaKmsUpdate     *update,
-                                                       MetaKmsUpdateFlag  flags);
+                                                       MetaKmsUpdateFlag  flags)
+  G_GNUC_WARN_UNUSED_RESULT;
 
 META_EXPORT_TEST
 void meta_kms_device_disable (MetaKmsDevice *device);

src/backends/native/meta-kms-update.h

@@ -81,6 +81,7 @@ typedef struct _MetaKmsPlaneFeedback
 typedef void (* MetaKmsResultListenerFunc) (const MetaKmsFeedback *feedback,
                                             gpointer               user_data);
 
+META_EXPORT_TEST
 void meta_kms_feedback_free (MetaKmsFeedback *feedback);
 
 MetaKmsFeedbackResult meta_kms_feedback_get_result (const MetaKmsFeedback *feedback);