dns: return real system error instead of 'ENOTFOUND'
Return the real system error to user insetad of injecting 'ENOTFOUND' which is not a proper POSIX error.
https://github.com/nodejs/node/pull/5099#issuecomment-181538896
Also fix the test suite to check for the corresponding error message.
The hostname '...' was replaced with '***' because '...' returns inconsistent error message on different system. On GNU libc it intantly returns EAI_NONAME but on musl libc it returns EAI_AGAIN after a timeout.