test: remove 512 bits test in test-tls-dhe.js

Test of 512 bits key is failed after upgrading openssl-1.0.1o due to
its limit of 768 bits key size. Remove it and start from 1024 bits
test.

Reviewed-By: Michael Dawson <mdawson@devrus.com>
PR-URL: https://github.com/joyent/node/pull/25533