private static void registerKeyOnly(Object key) {
    Region r = cache.getRegion(SEPARATOR + REGION_NAME);
    assertNotNull(r);
    r.registerInterest(key);
  }
wait for queues to drain in the server
geode-core/src/distributedTest/java/org/apache/geode/internal/cache/tier/sockets/InterestListDUnitTest.java