Kill reparented xcodebuild invocations from prior test launches
Kill reparented xcodebuild invocations from prior test launches

Summary: Fixes #384. This will ensure that any `xcodebuild` from an `FBDeviceXCTestCommands` invocation is killed on the next invocation. This is done by stashing an environment variable in the process, that can be fetched on the next invocation.

Reviewed By: marekcirkos

Differential Revision: D4627172

fbshipit-source-id: 894ff3e1e5e86a54fe919ffa530e2de18e181593

plu/FBSimulatorControl,StevenUpForever/FBSimulatorControl,plu/FBSimulatorControl,facebook/FBSimulatorControl,StevenUpForever/FBSimulatorControl,StevenUpForever/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,plu/FBSimulatorControl,facebook/FBSimulatorControl,StevenUpForever/FBSimulatorControl,plu/FBSimulatorControl