Add testing for the `req` app and explicit conversion tests similar to
what is done for ECDSA keys.
The included test keys for Ed25519 are from the examples in RFC 8410
(Sec. 10)
The key for Ed448 is derived from the first of the test vectors in
RFC 8032 (Sec. 7.4) using OpenSSL to encode it into PEM format.
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10410)
