test.py: fix broken receive of messages with multiple sinks

Problem:
With multiple sinks, more than one sink can receive first message due to
flooding in the beginning. receive() was called for each expected message,
on all sockets, possibly missing one.

Solution:
Check each sink for any amount of messages, read them all. Also there's
no need for select() timeouts.

Q: why multiple sources/sinks anyway? Doesn't make sense to me.
meshnode.py: fix crash on error

Without fix:
  File "./meshnode.py", line 115, in handle_packet
    print >> stderr, Exception("Bestätigung für Paket %s jedoch kein Paket vorhanden" % packet_id)
  exceptions.NameError: global name 'stderr' is not defined