I've contacted with Sitecore support for this and they have confirmed there is no out of the box report that retrieves detailed statistics for a/b test variants and registered a feature request to add it in future versions of Sitecore.
In the meantime, we can retrieve this data directly from the collection database. The information about the a/b variant is stored in the "Events" row of the "Interactions" table. It is called: "TestValueIndex". So the example of the query that for example counts the send interactions for "B" variant of the "'e6889dfb-ea7b-4393-8844-a56c29fc55fc'" email campaign will look like that:
SELECT COUNT(*) FROM [xdb_collection].[Interactions]
OdataType varchar(200) '$."@odata.type"',
MessageId varchar(200) '$.MessageId',
TestValueIndex varchar(200) '$.TestValueIndex'
OdataType = '#Sitecore.EmailCampaign.Model.XConnect.Events.EmailSentEvent'
MessageId = 'e6889dfb-ea7b-4393-8844-a56c29fc55fc'
TestValueIndex = '1'
To get full results, such a query needs to be applied for both shards.