GEODE-8951: Unnecessary messaging in WAN locator discovery

[alb3rtobr]

The ticket states that when a locator joins the cluster, it is notified about itself joining the system.
That problem was already solved by GEODE-6977, in locatorJoined(...) method of LocatorMembershipListenerImpl class, before starting the thread that will send the notifications about the new locator:

    // Remove locators that don't need to be notified (myself, the joining one and the one that
    // notified myself).
    List<DistributionLocatorId> ignoreList = Arrays.asList(locator, localLocatorId, sourceLocator);
    for (Map.Entry<Integer, Set<DistributionLocatorId>> entry : localCopy.entrySet()) {
      for (DistributionLocatorId removeLocId : ignoreList) {
        entry.getValue().remove(removeLocId);
      }
    }

Anyway, in order to verify that no more than the expected number of notification messages are sent, a check can be added to three unit tests of LocatorMembershipListenerImpl class. These test cases verify that the proper messages are sent, but they would not fail if unexpected messages were sent.

[alb3rtobr]

Kind reminder, review by code owners still pending

[alb3rtobr]

thanks @boglesby !