Fix FBTaskTest Race
Fix FBTaskTest Race

Summary: Due to the usage of queues here, the exitCode future will resolve before the `completed` since there is also some teardown. I want to eventually fix this for good by making asynchronous teardown a condition that occurs *after* the process has terminated, but for now this at least fixes the test

Reviewed By: zeyadsalloum

Differential Revision: D13564598

fbshipit-source-id: 5701365066c5109d076a56124a54cb76779841ca

facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl