Skip tls-crypt unit tests if required crypto mode not supported

Skip tls-crypt unit tests if required crypto mode not supported

Instead of failing the test with an unclear error, print that the a
required crypto primitive is not supported and skip the test.

This is for example the case when using the system-supplied openssl on
SLES11, which does not support AES-256-CTR.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1494859483-16466-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14657.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>