Port the long TCP stream test to Python

Port the long TCP stream test to Python

Previously, the packet.pl script was used to send the a series of frames
to named; this worked by accident as most of these were refused by the
kernel with EAGAIN. packet.pl prints a dot every 1000 packets, so this
slowed the script down and allowed some frames to get through.

Reimplement the test in Python: build the packet with dnspython, send
~6 MiB of data over TCP discarding all replies and then check if the
server is still alive.