Using Octoprint 1.1.1-14-g87ddcac and 1.2.5-dev, found that I was getting an HTTP 405 error (Method not allowed) when trying to "TEST". However, actually sending gcode to octoprint server works fine.
The Bonjour discovery isn't working either, but I don't know if I can pin that one on slic3r. Well, it's detecting my Lexmark E260dn, but not my octoprint server.
The "Test" button is POSTing to the /api/version resource instead of GETing, which is the reason for this problem. See here and here
Fix for issue #2526, changed to GET.
Thank you, fixed by @lordofhyphens