RFR: 8332535: Mark java/nio/channels/DatagramChannel/Loopback.java as intermittent [v3]

[SendaoYan]

> Hi all,
>   The test `test/jdk/java/nio/channels/DatagramChannel/Loopback.java` intermittent fail, the failure probability is about 2/1000. This PR mark this test intermittent, and print more messages to investigate the intermittent failure. Only change the testcase, no risk.

SendaoYan has updated the pull request incrementally with two additional commits since the last revision:

 - Merge branch 'jbs8332535' of github.com:sendaoYan/jdk-ysd into jbs8332535
 - add some entropy to the message content and discard any message that we receive whose payload doesn't match what we expect

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/19314/files
  - new: https://git.openjdk.org/jdk/pull/19314/files/3f4d450e..7bce58c1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19314&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19314&range=01-02

  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19314.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19314/head:pull/19314

PR: https://git.openjdk.org/jdk/pull/19314