Split up test-domain into multiple, more focused test files
and use more modern JS inside of them.
PR-URL: https://github.com/nodejs/node/pull/13614
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
