[API] Add hb_face_make/is_immutable()
diff --git a/test/test-font.c b/test/test-font.c
index 9c7d3e4..1b68834 100644
--- a/test/test-font.c
+++ b/test/test-font.c
@@ -156,7 +156,9 @@
   blob = hb_blob_create (test_data, sizeof (test_data), HB_MEMORY_MODE_READONLY, NULL, NULL);
   face = hb_face_create (blob, 0);
   hb_blob_destroy (blob);
+  g_assert (!hb_face_is_immutable (face));
   font = hb_font_create (face);
+  g_assert (hb_face_is_immutable (face));
   hb_face_destroy (face);