ARTEMIS-4985 message priority occasionally broken

The test in this commit was distilled down from a much more complex
integration test that rarely reproduced the problem. It is short and
sweet and reproduces the problem every time.

The problem exists in the iterator's `remove()` method where it uses
`index` instead of `i` when calculating a new highest priority.

artemis-commons/src/main/java/org/apache/activemq/artemis/utils/collections/PriorityLinkedListImpl.java

@@ -295,14 +295,8 @@ public class PriorityLinkedListImpl<E> implements PriorityLinkedList<E> {
 
          lastIter.remove();
 
-         // This next statement would be the equivalent of:
-         // if (index == highestPriority && levels[index].size() == 0)
-         // However we have to keep checking all the previous levels
-         // otherwise we would cache a max that will not exist
-         // what would make us eventually having hasNext() returning false
-         // as a bug
-         // Part of the fix for HORNETQ-705
-         for (int i = index; i >= 0 && levels[index].size() == 0; i--) {
+         // If the last message in the current priority is removed then find the next highest
+         for (int i = index; i >= 0 && levels[i].size() == 0; i--) {
             highestPriority = i;
          }
 

artemis-server/src/test/java/org/apache/activemq/artemis/core/list/PriorityLinkedListTest.java

@@ -881,6 +881,18 @@ public final class PriorityLinkedListTest {
       iter.remove();
    }
 
+   @Test
+   public void testMixupIterator3() {
+      list.addTail(b, 4);
+      list.addTail(c, 9);
+      LinkedListIterator<Wibble> iter = list.iterator();
+      assertTrue(iter.hasNext());
+      assertEquals(c, iter.next());
+      iter.remove();
+      list.addTail(a, 0);
+      assertTrue(iter.hasNext());
+      assertEquals(b, iter.next());
+   }
 
    @Test
    public void testPeek() {