Add --tls-crypt unit tests

Add --tls-crypt unit tests

These help verify the tls-crypt functionality - they already caught a
bug during development.  We should however probably also add some
t_client tests once this feature is in.

To test --tls-crypt with as few dependencies as possible, this adds a
mock implementation of msg() (or actually x_msg()).  For debugging
purposes, the mock implementation can be made to really log by calling
mock_set_debug_level(), but defaults to (almost) no logging.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1478636302-9678-6-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12973.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>