I’m actually not sure if that’s currently a feature available on Medium — Might be a good question for the Medium support team though.
What I’ve been doing on here is just re-editing the piece with a different headline if the article doesn’t seem to be getting traction. Only, that doesn’t always do the trick since it needs to get some engagement within the first initial days of being published.
I do, however, know of platforms that allow A/B testing specifically in relation to publishers I’ve worked with.
Sorry that may not have been extremely helpful, but if you find an alternative, do let me know!