[9]RFR 8130394: DatagramChannel tests need to be hardened to ignore stray datagrams

[FELIX YANG]

Hi all,
      please help to review the change for 8130394.

Issue:     https://bugs.openjdk.java.net/browse/JDK-8130394
Patch:    http://cr.openjdk.java.net/~fyuan/felix/8130394/

The patch updates tests to ignore stray datagrams, or at least print 
more information to ease troubleshooting.

thanks,
-Felix