add POSTINIT_CMD_suf to t_client.sh and sample config

add POSTINIT_CMD_suf to t_client.sh and sample config

We have pre-init and cleanup commands, but some test cases might need
or want to run a shell script after openvpn has initialized, but before
executing any tests (ifconfig comparison and ping).

Example: POSTINIT_CMD_4="sleep 5" on MacOS X for tap tests (IPv6 DAD)

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20161004113854.42470-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12594.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit bae1ad7005fd9a1fadeed56370a9ac5422a33fee)