[set] add a test demonstrating a bug in hb_set_union.

test/api/test-set.c

@@ -137,10 +137,13 @@ test_set_algebra (void)
 {
   hb_set_t *s = hb_set_create ();
   hb_set_t *o = hb_set_create ();
+  hb_set_t *o2 = hb_set_create ();
 
   hb_set_add (o, 13);
   hb_set_add (o, 19);
 
+  hb_set_add (o2, 0x660E);
+
   test_empty (s);
   g_assert (!hb_set_is_equal (s, o));
   hb_set_set (s, o);
@@ -157,6 +160,13 @@ test_set_algebra (void)
   g_assert (hb_set_has (s, 10));
   g_assert (hb_set_has (s, 13));
 
+  hb_set_clear (s);
+  test_empty (s);
+  g_assert_cmpint (hb_set_get_population (s), ==, 0);
+  hb_set_union (s, o2);
+  g_assert_cmpint (hb_set_get_population (s), ==, 1);
+  g_assert (hb_set_has (s, 0x660E));
+
   hb_set_clear (s);
   test_empty (s);
   hb_set_add_range (s, 10, 17);