test: refactor and fix test-dns

* More precise length assertion.
* Fix incorrect use of string instead of RegExp in `throws` assertions.
* Add missing RegExp to `throws` assertions.

PR-URL: https://github.com/nodejs/node/pull/9811


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>