[jdk11u-dev] RFR: 8273438: Enable parallelism in vmTestbase/metaspace/stressHierarchy tests

[Aleksey Shipilev]

Clean backport to improve testing performance.

Additional testing:
 - [x] Linux x86_64 fastdebug `vmTestbase/metaspace/stressHierarchy` passes

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Commit messages:
 - Backport a143372818ffab635b0e97208be53569b159a98b

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/779/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=779&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273438
  Stats: 15 lines in 15 files changed: 0 ins; 15 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/779.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/779/head:pull/779

PR: https://git.openjdk.java.net/jdk11u-dev/pull/779