Conscrypt relies on both this working at the EC_KEY level, but also EVP_PKEY. Make sure this keeps working, even as we mess around with EVP_PKEY's EC bits to separate the curves out. While I'm here, move the common HexToBIGNUM wrapper into test_util.h. Bug: 42290364 Change-Id: I58fa5a8487b8a499000a798040a53fc851b0a732 Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/81547 Reviewed-by: Lily Chen <chlily@google.com> Commit-Queue: David Benjamin <davidben@google.com>