tests/func: use rc4 cipher command for openssl

On ubuntu 13.04, _random() always cause segfault because of it triggers a bug
for aes-128-cbc cipher command.

By using rc4, it achieves the same throughput as aes-128-cbc and workaround the
segfault.

Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>