Fixed Enumerator.pushee test; wait on CountDownLatches more carefully

Fixed a timing issue with waiting on a CountDownLatch too early, which always then timed out. But the code didn't detect the fact that the CountDownLatch wait failed and the test continued anyway, causing an intermittent NPE (now happening reliably on my computer too, so perhaps a change elsewhere interacted with this test?).

Moved the wait to the right place, resolving the problem. Also wait longer and check return values for all calls to CountDownLatch.await().