tests/range: Add bug base URL

Every other unit test included a link to calendar's bug reporting except
test-range.c, so lets include that

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/820>

tests/test-range.c

@@ -300,6 +300,7 @@ main (gint   argc,
   g_setenv ("TZ", "UTC", TRUE);
 
   g_test_init (&argc, &argv, NULL);
+  g_test_bug_base ("https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/");
 
   g_test_add_func ("/range/new", range_new);
   g_test_add_func ("/range/invalid-ranges", range_invalid);