crypto: testmgr - add xts-aes-256 self-test

FIPS compliance requires a known-answer self-test for all approved
cipher and mode combinations, for all valid key sizes. Presently,
there are only self-tests for xts-aes-128. This adds a 256-bit one,
pulled from the same reference document, which should satisfy the
requirement.
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>