Xray actually supports timebomb licenses, however the following steps need to be performed:

Step-by-step guide

1. Head on to the darkfeatures menu of Jira. http://host_server/secure/SiteDarkFeatures!default.jspa
2. Add this feature com.xpandit.raven.license.impl.AtlassianLicenseManager.tm.test
   
3. Copy a timebomb license from https://developer.atlassian.com/platform/marketplace/timebomb-licenses-for-testing-server-apps/
4. Add it to Manage Apps Xray License
5. The license is now a trial ending at the end of the license
   

Link for related documentation

Timebomb licenses for testing server apps

• https://developer.atlassian.com/platform/marketplace/timebomb-licenses-for-testing-server-apps/

Licensing your Jira applications

• https://confluence.atlassian.com/adminjiraserver/licensing-your-jira-applications-938847718.html