test.py is broken sending multiple messages. fix it.

Problem:
Sending multiple messages in the test sends all packets with ID 0.
'meshnode.py' incorrectly passes the test while a correct implementation
drops the repeated packets with ID 0, only letting the first packet pass
and then fail the test.

Fix:
- Add optional 'packet_nr' argument to 'pipe' command in util.py, specifing
  the first packet ID to be sent. Being optional, 'pipe' usage remains
  backward compatile with whatever useage.
- Fix 'test.py' to use the new 'packet_nr' argument, using the message number
  as packet ID.

Todo: fix meshnode.py to behave correctly