[jdk11u-dev] RFR: 8305186: Reference.waitForReferenceProcessing should be more accessible to tests

[SendaoYan]

Hi all,

This pull request contains a backport of commit [d8f012ea](https://github.com/openjdk/jdk/commit/d8f012ea2a0514020434d5db6047e36941e9349b) from the [openjdk/jdk](https://git.openjdk.org/jdk) repository.

The commit being backported was authored by Brent Christian on 25 Apr 2025 and was reviewed by Kim Barrett.

Thanks!

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Commit messages:
 - Revert change of supportsConcurrentGCPhaseControl
 - Revert "Backport da4dd7c008da4be54ee5f58d1ac19f440fd2a74e"
 - Backport d8f012ea2a0514020434d5db6047e36941e9349b
 - Backport da4dd7c008da4be54ee5f58d1ac19f440fd2a74e

Changes: https://git.openjdk.org/jdk11u-dev/pull/3105/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=3105&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305186
  Stats: 51 lines in 1 file changed: 51 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/3105.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/3105/head:pull/3105

PR: https://git.openjdk.org/jdk11u-dev/pull/3105