cmocka: use relative paths

cmocka: use relative paths

Simplifies the build scripts, and fixes my CI, where paths on the test
slave can be different from paths on the build slave.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20181028145449.12676-1-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17849.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 4ded2deda0d6d33d38fbbd1f4cd4e9acc0836a89)