Actually wait for simulator shutdown future
Actually wait for simulator shutdown future

Summary: `finishedExecutionOnSimulator` returns FBFuture that chains all simulator shutdown. We should resolve it before returning.

Reviewed By: lawrencelomax

Differential Revision: D6396474

fbshipit-source-id: a9f44de2354c00e769a2f4b82b2b6382c060572c

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