Change ossltest engine to manually allocate cipher_data

The ossltest engine wraps the built-in implementation of aes128-cbc.
Normally in an engine the cipher_data structure is automatically allocated
by the EVP layer. However this relies on the engine specifying up front
the size of that cipher_data structure. In the case of ossltest this value
isn't available at compile time. This change makes the ossltest engine
allocate its own cipher_data structure instead of leaving it to the EVP
layer.

Reviewed-by: Andy Polyakov <appro@openssl.org>