tests/t_lpback.sh: Switch sed(1) to POSIX-compatible regex.

tests/t_lpback.sh: Switch sed(1) to POSIX-compatible regex.

A test run with FreeBSD PR 229925 'Disallow escaping ordinary
characters in regex(3)' reveals one sed expression that uses the
GNU-extension "\s".

Given that this is the only occurrence and it's a trivial fix,
update it to be POSIX-compatible.

Signed-off-by: Matthias Andree <matthias.andree@gmx.de>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20190906174458.14975-2-matthias.andree@gmx.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18806.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 7e4a261cc92a813f9e9ba9ee91c6e08de9d843f8)