A/B testing is built into the flow builder for both campaigns and automations.
You'll find it in the toolbox of the Flow Builder: drag and drop the node like any other component to set up your test scenarios.
You can split traffic, for example 30% to a first variant and 70% to a second one. Each branch can then be connected to templates, messages or any other element of your flow.
Version 1 of the feature tracks the number of clicks generated by each variant. A V2 is in development and will add conversion statistics for a finer analysis (e.g. fewer clicks but more conversions on one variant).