Test: link drbgtest statically against libcrypto

and remove duplicate rand_drbg_seedlen() implementation again.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7462)

(cherry picked from commit 1c615e4ce97715ae3af9255bc57be32a49687966)