test: add toASCII and toUnicode punycode tests

- Add toUnicode and toASCII tests to test-punycode
- Refactor test-punycode.js to better organize test cases
- Change assert.equal to assert.strictEqual in test-punycode.js

PR-URL: https://github.com/nodejs/node/pull/9741


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>