Some asymmetric algorithms will get different ciphertext after
each encryption, such as SM2, and let testmgr support the testing
of such algorithms.

In struct akcipher_testvec, set c and c_size to be empty, skip
the comparison of the ciphertext, and compare the decrypted
plaintext with m to achieve the test purpose.
Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Tested-by: Xufeng Zhang <yunbo.xufeng@linux.alibaba.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>