Test an Iterable ADTs

Let's add a simple test to the IndexedListTest to test the (inherited) IndexedList.iterator method:

@Test
@DisplayName("test iterator after IndexedList is instantiated.")
void testEnhancedLoopAfterConstruction() {
  int counter = 0;
  for(int element: numbers) {
    assertEquals(defaultValue, element);
    counter++;
  }
  assertEquals(size, counter);
}