Make test_killed_worker_recover faster

I3f7bd0ba5ef1bb716a97635252abe251053a669d changed how server.wait()
works so that it wouldn't return until all children are done. But
test_killed_worker_recover doesn't try to kill the children, unlike other
tests in test_multiprocess_api. This didn't cause the unit test to fail
because tearDown catches the unit test timeout, fixtures.TimeoutException,
which triggers after 160 seconds.

Now instead of taking 160 seconds test_killed_worker_recover, which is
run twice, takes under 3 seconds.

Change-Id: I8cbd08cb873cd81a65005764759ac9ca23fa6430
Closes-Bug: #1360719
Merge "Make test_killed_worker_recover faster"