lisa.wlgen.workload: Add missing wait() call in run()
lisa.wlgen.workload: Add missing wait() call in run()

Similar to Popen, devlib's BackgroundCommand.__exit__ will close
stdin/stdout/stderr of the process before waiting on it. Since this will
usually result in early termination of the workload, wait() on it
explicitly so that we gracefully wait for the command to finish before
closing its streams.

ARM-software/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,credp/lisa,ARM-software/lisa,credp/lisa