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.