Fix race condition in FBFuture test
Fix race condition in FBFuture test

Summary: Add critical section around `lines`.

Reviewed By: lawrencelomax

Differential Revision: D23977792

fbshipit-source-id: 231a355d28b5668d35a6d46416c0f60144471637

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