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

[Kim Barrett]

On Fri, 11 Apr 2025 23:31:16 GMT, Brent Christian <bchristi at openjdk.org> wrote:

>> 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

Changes requested by kbarrett (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/24527#pullrequestreview-2763075157