test: be explicit about polluting of `global`

There was a comment in `test-domain-crypto.js` indicating that the
pollution of the `global` object with a `domain` property was
intentional. Provide more information in the comment so someone may
easily determine why. Use `global.domain` rather than declaring `domain`
without the `var` keyword to more clearly signal that the pollution is
intentional.

PR-URL: https://github.com/nodejs/node/pull/6017


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>