test: add regression test for 512 bits DH key

Check that trying to use a < 1024 bits DH key throws an exception.

parallel/test-tls-dhe tests this as well but it feels incongruous not to
do it here when both tests have similar logic for 1024/2048 bits keys.

PR-URL: https://github.com/nodejs/node/pull/3629


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>