[jdk17u-dev] RFR: 8297936: Use reachabilityFence to manage liveness in ClassUnload tests

[Goetz Lindenmaier]

I backport this to make later backports clean.

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Commit messages:
 - Backport 5085bd5f05ca70e08c6764ed208d574c556b6c57

Changes: https://git.openjdk.org/jdk17u-dev/pull/3904/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=3904&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297936
  Stats: 23 lines in 3 files changed: 9 ins; 5 del; 9 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/3904.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/3904/head:pull/3904

PR: https://git.openjdk.org/jdk17u-dev/pull/3904