ObservationQueueFullWarnTest#testQueueFullThenFlushing requires to know the number of events upfront

The test ObservationQueueFullWarnTest#testQueueFullThenFlushing uses EventIterator#getSize but it always assumes that the method returns a positive value. Instead, the Javadoc for EventIterator#getSize states that the implementation must return -1 if there aren't any information about the precise number of elements. The test should be adapted in order to account for this corner case.