crypto: testmgr - Add ChaCha20 test vectors from RFC7539

We explicitly set the Initial block Counter by prepending it to the nonce in
Little Endian. The same test vector is used for both encryption and decryption,
ChaCha20 is a cipher XORing a keystream.

Signed-off-by: Martin Willi <martin@strongswan.org>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>