unit_tests: Add missing cert_data.h to source list for unit tests

unit_tests: Add missing cert_data.h to source list for unit tests

Document the dependency. Also fixes cert_data.h missing from
distribution.

This is the "backport" of commit
97223cb057a0edfafd28b34427449bb3eda7d0be to release/2.6.

Change-Id: Ib82208bfa12cc8ba5ff08b4c010bf398bc92d249
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230621125842.191355-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26765.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>