Are there any constantly updating RSS feed services to use for testing, or just very active RSS feeds?

I really needed this service, so I set one up at http://lorem-rss.herokuapp.com/

The documentation is at that link, as well as in the GitHub repository below. You can tweak it using GET parameters to update every N seconds, minutes, hours, days, months or years. But I haven't yet tested it comprehensively so I'm not sure if the RSS validates well across different parsers. YMMV.

The code is on GitHub at https://github.com/mbertolacci/lorem-rss

If you select a highly active feed (http://rss.cnn.com/rss/cnn_topstories.rss) then the updates should be very frequent.

Be aware that some sites use different RSS versions so you might have to adjust your code as required.