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

[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 message to investigate the intermittent failure. Only change the testcase, no risk.

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Commit messages:
 - 8332535: Mark java/nio/channels/DatagramChannel/Loopback.java as intermittent

Changes: https://git.openjdk.org/jdk/pull/19314/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19314&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332535
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 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