How to get a/b testing on the number of unique open?
I suspect this is a code change and should be made at the api level:

api/v3/MailingAB.php, function civicrm_api3_mailing_a_b_graph_stats

(which in turn calls function in the CRM_Mailing_Event_BAO_Opened class)

Would be good to fix the definition of opened as you suggest across both A/B stats and the reports since that is the accurate and valid number