Switch to scapy with python 3.

Do not use privileged or NFS source ports for UDP packets as inetd
ignores such packets. This should avoid some sporadic failures.
While there, use variable names consistently in all tests.

Print better error message if test fails.

By removing all the sleeps I created a race in the pf tests. The
packet was sent and its reply received before the sniffer was up
and running. So sleep a second after starting the sniffer thread
bu

By removing all the sleeps I created a race in the pf tests. The
packet was sent and its reply received before the sniffer was up
and running. So sleep a second after starting the sniffer thread
but before sending the packet. With pf_forward 0m51.54s, pf_fragment
1m33.59s, pf_state 0m03.64s the test are still faster than with
forking.

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Make the test faster. Move all the packet matching code into the
pcap filter. That means the first packet sniffed is the correct
one. In the success case, we can stop without waiting for a timeout

Make the test faster. Move all the packet matching code into the
pcap filter. That means the first packet sniffed is the correct
one. In the success case, we can stop without waiting for a timeout.
OK sashan@

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Add a test suite for pf state handling. It needs a remote machine
to exchange packets, the remote kernel is tested. For now it only
contains a challenge ack test written by sashan@.
OK sashan@