RFR: 8297936: Use reachabilityFence to manage liveness in ClassUnload tests [v2]

[Afshin Zafari]

> ### Description
> Instead of using static instances or using an object in print to keep it alive, `java.lang.ref.Reference.reachabilityFence(Object)` is used to keep an object alive and avoid class unloading.
> 
> ### Tests
> linux-x64-debug, macosx-aarch64-debug, mach5 tier1-5

Afshin Zafari has updated the pull request incrementally with one additional commit since the last revision:

  8297936: Use reachabilityFence to manage liveness in ClassUnload tests

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/12552/files
  - new: https://git.openjdk.org/jdk/pull/12552/files/e7c92018..c4abab68

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12552&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12552&range=00-01

  Stats: 7 lines in 3 files changed: 3 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12552.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12552/head:pull/12552

PR: https://git.openjdk.org/jdk/pull/12552