Add unit tests for engine keys

Add unit tests for engine keys

Testing engines is problematic, so one of the prerequisites built for
the tests is a simple openssl engine that reads a non-standard PEM
guarded key.  The test is simply can we run a client/server
configuration with the usual sample key replaced by an engine key.
The trivial engine prints out some operations and we check for these
in the log to make sure the engine was used to load the key and that
it correctly got the password.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200622232319.8143-2-James.Bottomley@HansenPartnership.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20075.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>