RFR: 8305186: Reference.waitForReferenceProcessing should be more accessible to tests [v6]

[Brent Christian]

> Certain specific types of tests involving GC and reference processing need to account for the delay between a GC completing (during which the GC clears a Reference), and the Reference being added to its associated queue. At present, ad hoc mechanisms (with delays/timeout) are used, but can lead to intermittent test failures ([JDK-8298783](https://bugs.openjdk.org/browse/JDK-8298783)  is a recent example).
> 
> A better mechanism already exists in the private `Reference.waitForReferenceProcessing()` method. This PR makes `waitForReferenceProcessing()` available to tests via the `WhiteBox` and `ForceGC` test libraries.

Brent Christian has updated the pull request incrementally with two additional commits since the last revision:

 - Make *the method* static, also
 - Make Method static

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24527/files
  - new: https://git.openjdk.org/jdk/pull/24527/files/58191e86..e47caf5a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24527&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24527&range=04-05

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24527.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24527/head:pull/24527

PR: https://git.openjdk.org/jdk/pull/24527