Minor t_client.sh cleanups

- remove built tests/t_client.sh script on "make clean"
- ignore Linux iproute2 "ssthresh <n>" output that sometimes shows up
  in "ip -6 route show" and breaks before/after comparison

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1398019261-30180-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8557
(cherry picked from commit 1e3a1786a80e4afac37133ce5d6a1dcff779a4ce)