Add Unit test for environment output
Add Unit test for environment output

Summary: Found a bug here in the `posix_spawn` implementation, writing a test to cover it

Reviewed By: c-ryan747

Differential Revision: D15621325

fbshipit-source-id: 30238b5e5b2bc184409070d21e6925a4ac970f15

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