Use Xvfb for testing

Jenkinsfile

@@ -30,9 +30,11 @@ pipeline {
             }
         }
         stage("Test") {
+            wrap([$class: 'Xvfb']) {
                 steps {
                     sh "mvn test"
                 }
+            }
             post {
                 success {
                     echo "Generating Test Report..."