zdtm.sh: dump traffic for *tcpbuf* tests

Sometimes these tests don't pass successfully and we need more
information to investigate the problem.

https://bugzilla.openvz.org/show_bug.cgi?id=2676


Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>