RFR: 8332535: Discard any message not expect in Loopback.java [v7]

[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 one additional commit since the last revision:

  delete @key intermittent

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/19314/files
  - new: https://git.openjdk.org/jdk/pull/19314/files/7b06dcf1..ad8e1de0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19314&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19314&range=05-06

  Stats: 2 lines in 1 file changed: 0 ins; 1 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