Add predicatable RAND_METHOD to test ENGINE

The test ENGINE effectively used a predictable PRNG because it supplied
a bogus implementation of SHA256 which the old version of OpenSSL's PRNG
used. The new DRBG does not use SHA256 so it is no longer predictable
if the SHA256 implementation is replaced. Use an explicit predictable
PRNG instead.

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/4098)