@Override
    public void quorumLost(DistributionManager distributionManager,
        Set<InternalDistributedMember> failures,
        List<InternalDistributedMember> remaining) {
      quorumLostInvoked = true;
      logger.info("quorumLost invoked in test code");
    }
New test hook which blocks before closing channel.
geode-core/src/distributedTest/java/org/apache/geode/distributed/LocatorDUnitTest.java