Add nsupdate timeout tests

Add nsupdate timeout tests

* nsupdate should take 12 seconds (one try and three retries with
  3 second timeout for each), UDP mode
* nsupdate -u 4 -r 1 should take 8 seconds (one try and one retry with
  4 second timeout for each), UDP mode
* nsupdate -u 0 -t 8 -r 1 should also take 8 seconds, UDP mode
* nsupdate -u 4 -t 30 -r 1 should also take 8 seconds, as -u takes
  precedence over -t, UDP mode
* nsupdate -t 8 -v should also take 8 seconds, TCP mode