WaitQueueTest is flakey

While the test is a little noddy, and superfluous, it shouldn't fail even vanishingly infrequently. aweisberg has spotted it doing so, and I have also encountered it once, so I suspect that a change in hardware/OS may have made vanishingly unlikely just pretty unlikely, which is even less good.

Right now it depends on Thread.start() completing before the new thread starts; this isn't guaranteed. This patch fixes that.