[jdk21u-dev] RFR: 8323670: A few client tests intermittently throw ConcurrentModificationException

[Martin Doerr]

Clean backport of [JDK-8323670](https://bugs.openjdk.org/browse/JDK-8323670).

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Commit messages:
 - Backport 70e7cdcb9ecedd455cf701b5c7ad05286ac0067d

Changes: https://git.openjdk.org/jdk21u-dev/pull/620/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=620&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323670
  Stats: 48 lines in 1 file changed: 6 ins; 4 del; 38 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/620.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/620/head:pull/620

PR: https://git.openjdk.org/jdk21u-dev/pull/620