crypto: testmgr - add chunked test cases for chacha20

We failed to catch a bug in the chacha20 code after porting it to the
skcipher API. We would have caught it if any chunked tests had been
defined, so define some now so we will catch future regressions.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>