Use a single local port for ditch.pl

Use a single local port for ditch.pl

The ditch.pl script is used to generate burst traffic without waiting
for the responses. When running other tests in parallel, this can result
in a ephemeral port clash, since the ditch.pl process closes the socket
immediately. In rare occasions when the message ID also clashes with
other tests' queries, it might result in an UnexpectedSource error from
dnspython.

Use a dedicated port EXTRAPORT8 which is reserved for each test as a
source port for the burst traffic.