test: improve punycode test coverage

Adds two additional tests for mapDomain function in punycode.js.
When an email address is given to the toASCII() and toUnicode()
functions, only the parts before the '@' character should be
encoded/decoded.

PR-URL: https://github.com/nodejs/node/pull/11144


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>