Avoid linking libcrypto in the valgrind ct test.

Summary:
Libcrypto isn't useful here and on some systems UB in OpenSSL's
init causes failures.

Fixes #775.

This is a backport of secp256k1 PR795

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7599