[jdk21u-dev] RFR: 8318727: Enable parallelism in vmTestbase/vm/gc/concurrent tests

[Elif Aslan]

Clean backport. Testing is in progress

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Commit messages:
 - Backport 29d462a07239a57b83850b9a8662573291fdbdf7

Changes: https://git.openjdk.org/jdk21u-dev/pull/416/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=416&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318727
  Stats: 943 lines in 41 files changed: 0 ins; 943 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/416.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/416/head:pull/416

PR: https://git.openjdk.org/jdk21u-dev/pull/416