Add test_ssl unit test and test export of PEM to file

Add test_ssl unit test and test export of PEM to file

This introduces a number of mock function to be able to compile
ssl_verify_*.c and ssl_mbedtls.c/ssl_openssl.c into a unit and adds
quite a number of files to that unit. But it allows similar unit tests
(in term of dependencies) to be added in the future.

Change-Id: Ie248d35d063bb6878f3dd42840c77ba0d6fa3381
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20240116214152.27316-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28028.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>