Add missing mixnet test to integration runs (#414)

ci/Jenkinsfile.nightly.integration

@@ -39,12 +39,21 @@ pipeline {
           }
         }
         stages {
-          stage('BuildAndTest') {
+          stage('Tests') {
+            options {
+              lock('sync-integration-${env.GIT_COMMIT}')
+            }
+            stages {
+              stage("BuildAndTest") {
                 steps {
                   script {
                     /* To prevent rebuilding node for each test, tests are defined here */
                     def tests = ['ten_nodes_happy', 'two_nodes_happy', 'ten_nodes_one_down']
 
+                    if (FEATURE == 'libp2p') {
+                      tests.add('mixnet')
+                    }
+
                     runBuildAndTestsForFeature(FEATURE, tests)
                   }
                 }
@@ -53,6 +62,8 @@ pipeline {
           }
         }
       }
+    }
+  }
 
   post {
     failure {