Fix test-set leak issue

See also https://github.com/harfbuzz/harfbuzz/pull/1169

test/api/test-set.c

diff --git a/test/api/test-set.c b/test/api/test-set.c
index 338a610..eb2f22e 100644
--- a/test/api/test-set.c
+++ b/test/api/test-set.c
@@ -262,6 +262,7 @@
 
   hb_set_destroy (s);
   hb_set_destroy (o);
+  hb_set_destroy (o2);
 }
 
 static void