[secp256k1] Support OpenSSL versions >= 1.1 for ENABLE_OPENSSL_TESTS

Summary: Backport PR494 from Alexander Block

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1257