net-test: Adds the --dry_run flag to packetdrill to verify but not start a run.

The --dry_run command line flag causes packetdrill to verify command
line args, load and parse the given script, but not execute it. If any
errors are found during these steps, packetdrill will print an error
as before.  Otherwise, it will exit(0) and produce no output.

Change-Id: Ibf4502c6ea326539346a58b58d7feb29d4b3da69