Add OSSLTest Engine

This engine is for testing purposes only. It provides crippled crypto
implementations and therefore must not be used in any instance where
security is required.

This will be used by the forthcoming libssl test harness which will operate
as a man-in-the-middle proxy. The test harness will be able to modify
TLS packets and read their contents. By using this test engine packets are
not encrypted and MAC codes always verify.

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