Import C2SP/CCTV test

Import C2SP/CCTV test

This currently only covers Ed25519 using the c2sp-testvectors package
and checks that our Ed25519 implementation behaves as expected from a
"ref10" implementation.

This test has Go and c2sp-testvectors as a hard dependency. It will
optionally pick up any OpenSSL package installed on the system and
test that as well.

https://github.com/C2SP/CCTV
https://github.com/C2SP/CCTV/tree/main/ed25519