I've implemented a minimal NTP server that doesn't actually adjust the time but responds to NTP datagram requests with info filled out using the current clock time to be be used for debugging and testing. At minimum it can be used as a test harness to test the NTP client implementations such as the NtpClient class. Added appropriate junit test.
The SimpleNTPServer can run from any local port to prevent from interfering with the actual NTP service on port 123.
The SimpleNTPServer class complements the existing NtpClient class.
Implementation and test provided in attached patch file.