Remove use of BN_init, BN_RECP_CTX_init from bntest

BN_init and BN_RECP_CTX_init are deprecated and are not exported
from shared libraries on some platforms (e.g. Windows) convert
bntest to use BN_new and BN_RECP_CTX_new instead.
Reviewed-by: Matt Caswell <matt@openssl.org>