Add unit test for encrypting/decrypting data channel

Add unit test for encrypting/decrypting data channel

This test is reusing code from --test-crypto but is modified to not rely
on the static key functionality and also only tests the most common
algorithm. So it does not yet completely replace --test-crypto

Change-Id: Ifa5ae96165d17b3cae4afc53e844bb34d1610e58
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20240208085749.869-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28195.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>