Is it possible to apply other segments (such as Country or Marketing campaign) to the A/B tests reports?

Yes, you can apply any Matomo (Piwik) segments to your A/B tests reports and see for example how your different variations compare against each other in different countries (when applying the “Country” segment), or how your variations compare depending on which marketing campaign or referrer brought the visitor to your website or app. To do this open an A/B test report, then create or apply your custom segment.