I'd recommend using programs within your engagement stream and a smart campaign to define your members and the A/B flow steps. Programs open up another world of fun you wouldn't really be able to do otherwise.
How I structured my A/B testing is by cloning the email and making my changes on the clone. Using the native A/B test feature within Engagement programs has caused problems. There's disucssions on this throughout the community.
Within your flow step of the smart campaign, use the random sample restrain to define your split.
After your program is all set place it into the nurture and when prompted, select your Smart Campaign you just built to ensure your email gets pulled into the native engagement program send.
I have screenshots if you need.