Add a libssl test harness

This commit provides a set of perl modules that support the testing of
libssl. The test harness operates as a man-in-the-middle proxy between
s_server and s_client. Both s_server and s_client must be started using the
"-testmode" option which loads the new OSSLTEST engine.

The test harness enables scripts to be written that can examine the packets
sent during a handshake, as well as (potentially) modifying them so that
otherwise illegal handshake messages can be sent.

Reviewed-by: Richard Levitte <levitte@openssl.org>