Add test for hb_set_del

test/api/test-set.c

diff --git a/test/api/test-set.c b/test/api/test-set.c
index 1382ada..aa2b388 100644
--- a/test/api/test-set.c
+++ b/test/api/test-set.c
@@ -118,6 +118,9 @@
   g_assert (!hb_set_has (s, 801));
   g_assert (!hb_set_has (s, 802));
 
+  hb_set_del (s, 800);
+  g_assert (!hb_set_has (s, 800));
+
   hb_set_destroy (s);
 }