[jdk21u] RFR: 8315774: Enable parallelism in vmTestbase/gc/g1/unloading tests

[Aleksey Shipilev]

Clean backport to improve testing times. The change was in mainline for about 7 weeks, without bugtail.

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Commit messages:
 - Backport 825e0ed2a1fde617ad2a7b74d951df3c1417d463

Changes: https://git.openjdk.org/jdk21u/pull/296/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=296&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315774
  Stats: 2576 lines in 112 files changed: 0 ins; 2576 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u/pull/296.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/296/head:pull/296

PR: https://git.openjdk.org/jdk21u/pull/296