[SECP256K1] Enable the OpenSSL tests (and benchmark)

Summary:
This diff enables building the OpenSSL tests by default with CMake. In
order to get rid of the dependency, the SECP256K1_BUILD_OPENSSL_TESTS
variable can be set to OFF. It will also be disabled if the tests are
disabled.

Test Plan:

cmake -GNinja .. -DSECP256K1_BUILD_TEST=OFF
grep SECP256K1_BUILD_OPENSSL_TESTS CMakeCache.txt

Check that SECP256K1_BUILD_OPENSSL_TESTS is not defined in the cache.

cmake -GNinja .. -DSECP256K1_BUILD_OPENSSL_TESTS=OFF
grep SECP256K1_BUILD_OPENSSL_TESTS CMakeCache.txt

Check that SECP256K1_BUILD_OPENSSL_TESTS is disabled in the cache.

cmake -GNinja ..
grep SECP256K1_BUILD_OPENSSL_TESTS CMakeCache.txt

Check that SECP256K1_BUILD_OPENSSL_TESTS is enabled in the cache.

ninja check-secp256k1
ninja bench-secp256k1

Check in the output that the benchmark ecdsa_verify_openssl is run.

Run the Travis build (see
https://travis-ci.org/Fabcien/secp256k1/builds/657808023).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5429