crypto: curve25519 - add kpp selftest

In preparation of introducing KPP implementations of Curve25519, import
the set of test cases proposed by the Zinc patch set, but converted to
the KPP format.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>