Update java/util/prefs/AddNodeChangeListener.java to report more failure info

java/util/prefs/AddNodeChangeListener.java fails intermittently, but the failure is hard to reproduce.

The test should be updated to provide more information when the test fails. Hopefully the next time the test fails, the cause of the failure can be deduced.

Useful information might include:
* specifics about which part of the test fails
* whether the sleep() calls need to sleep for longer