Does this only happen on Cygwin buildbots ?
If yes, then it might simply be an issue with Cygwin's fork implementation, which is much slower than natively.
Right now, the test waits 0.5s before checking that the processes are started, after repopulating the pool. While 0.5s is normally way enough for forking a couple processes, it seems that under Cygwin this can take a surprising amount of time, see http://old.nabble.com/Slow-fork-issue---Win-x64-td19538601.html and also http://superuser.com/questions/133313/can-i-speed-up-cygwins-fork
Unless I misunderstand their benchmarks, the fork (+exec) rate of "date" from a shell can be as low as 5/sec, so I can only guess what forking cpython would take.
Maybe we could try to increase the timeout before checking the PIDs:

+    countdown = 10
-    countdown = 10
    while countdown and not all(w.is_alive() for w in p._pool):
        countdown -= 1
        time.sleep(DELTA)