Add argv_insert_head__empty_argv__head_only to argv tests

Add argv_insert_head__empty_argv__head_only to argv tests

The unit test argv_insert_head__empty_argv__head_only was defined
but never used. Add it to the array of unit tests.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20211208170614.3404821-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23359.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit cc39fc7f3bddc6567507406cf8a389418e101bf8)