RFR: 8262049: [TESTBUG] Fix TestReferenceRefersTo.java for Shenandoah IU mode [v6]

[Kim Barrett]

On Tue, 23 Feb 2021 08:44:58 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> Shenandoah's IU mode allows referents to be cleared even when accessed during concurrent marking. The test TestReferenceRefersTo.java needs to be adjusted to allow for that.
>> 
>> Test:
>>  - [x] TestReferenceRefersTo.java + Shenandoah/IU
>>  - [x] TestReferenceRefersTo.java + Shenandoah/SATB
>>  - [x] TestReferenceRefersTo.java + G1
>
> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Allow Shenandoah for TestReferenceRefersToDuringConcMark test, except IU mode

Marked as reviewed by kbarrett (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/2653