The execwrapper package wraps the exec.Cmd and has a Command
interface that mimics the behaviour of the exec.Cmd.
This is useful for testing the lazyfilewalker subprocesses
by stubbing instead of spawning a real subprocess.
Updates https://github.com/storj/storj/issues/5349
Change-Id: I14084139c76a531f2b6d7163f9aa35c3f5e192d7