When I go through the Create new A/B test workflow, it presents me with a traditional mailing interface, even though I have Mosaico installed.
I'm pretty sure that means A/B testing is not possible with Mosaico but wanted to check for confirmation.
A/B testing for subjects is now possible (though not content or from details), as of Mosaico CiviCRM 2.6.
According to a 2017 blog post: no, it's not implemented yet.
I think it should probably be opened as an issue on github.