ISPN-4544 testCombinerForDistributedReduction issue

Master only. Will should review. See https://issues.jboss.org/browse/ISPN-4544 for more details
No no, the default collector size is 1000 so combine is never invoked for small tasks! If combine is never invoked we can not test properly. However, we use queue of collectors only for distributed map/reduce (large tasks) otherwise we use simpler algorithm that invokes combine always. Hence this test is a bit convoluted.