test: add script to create 0-dns-cert.pem

0-dns-cert.pem  and 0-dns-key.pem were stored in `test/fixtures/key`
directory, but the cert file cannot be created with the openssl
command via Makefile.

Added a script to create it with using  `asn1.js` and
`asn1.js-rfc5280` and moved them out of key directory and put into
`test/fixtures/0-dns`.

The domains listed in the cert were also changed into example.com and
example.org to show the use for only testing.

Fixes: https://github.com/nodejs/node/issues/10228
PR-URL: https://github.com/nodejs/node/pull/11579


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>