crypto: lib/curve25519 - re-add selftests

Somehow these were dropped when Zinc was being integrated, which is
problematic, because testing the library interface for Curve25519 is
important.. This commit simply adds them back and wires them in in the
same way that the blake2s selftests are wired in.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>