build: fix OpenSSL EC detection on macOS

Summary:

• build: add SECP_TEST_INCLUDES to bench_verify CPPFLAGS

This is needed so that bench_verify gets CRYPTO_CPPFLAGS, which would
otherwise not be included, at least on macOS.

This is a backport of linsecp256k1 PR735

Test Plan: I don't have an OSX machine, so \o/

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7583