@After
  public void tearDown() {
    for (VM vm : asList(vm4, vm5, vm6, vm7)) {
      vm.invoke(() -> DistributionMessageObserver.setInstance(null));
    }
  }
DUnit test for operations on ParallelGatewaySender
geode-wan/src/distributedTest/java/org/apache/geode/internal/cache/wan/parallel/ParallelGatewaySenderOperationsDistributedTest.java