[jdk11u-dev] RFR: 8315774: Enable parallelism in vmTestbase/gc/g1/unloading tests

[Daniel Hu]

Clean backport to make vmTestbase/gc/g1/unloading tests parallel. Passes GHA and vmTestbase/gc/g1/unloading tests.

`time make run-test TEST="vmTestbase/gc/g1/unloading`
Before:
`1220.85s user 109.96s system 53% cpu 41:28.83 total`
After:
`1240.52s user 89.00s system 707% cpu 3:07.93 total`

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Commit messages:
 - Backport 825e0ed2a1fde617ad2a7b74d951df3c1417d463

Changes: https://git.openjdk.org/jdk11u-dev/pull/3044/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=3044&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/jdk11u-dev/pull/3044.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/3044/head:pull/3044

PR: https://git.openjdk.org/jdk11u-dev/pull/3044