make t_client robust against sudoers misconfiguration

make t_client robust against sudoers misconfiguration

Instead of testing (and priming) sudo with "true", prime with
"kill -0 $$" (just test signalling ourselves).  If this fails,
we won't be able to kill the openvpn process we're going to
start later on -> thus, SKIP on failure.

This helps with misconfigured setups (especially on the buildbots)
that can correctly start openvpn but then not stop it later on -
leaving openvpn processes dangling around, requiring manual
intervention.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20161002131923.36681-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12585.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 8ca29af7c6d4759ce019ec9d0cd3eae4511a6804)