test: check curve algorithm is supported

parallel/test-crypto-dh.js assumes particular curve algorithms
(e.g. Oakley-EC2N-3) are supported, though this may not necessarily be
the case if Node.js was built with a system version of OpenSSL.

PR-URL: https://github.com/nodejs/node/pull/12265


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>