crypto: xchacha - add test vector from XChaCha20 draft RFC

There is a draft specification for XChaCha20 being worked on.  Add the
XChaCha20 test vector from the appendix so that we can be extra sure the
kernel's implementation is compatible.

I also recomputed the ciphertext with XChaCha12 and added it there too,
to keep the tests for XChaCha20 and XChaCha12 in sync.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>